Moves Firebase notifications to a "google" build flavor and adds an "fdroid" flavor that uses polling for notifications.

Also, adds a few missing license notices.

app/src/fdroid/AndroidManifest.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="com.keylesspalace.tusky">
+
+    <application>
+        <service
+            android:name=".MessagingService"
+            android:enabled="true"
+            android:exported="true" />
+    </application>
+
+</manifest>

app/src/fdroid/java/com/keylesspalace/tusky/MessagingService.java

@@ -0,0 +1,114 @@
+/* Copyright 2017 Andrew Dawson
+ *
+ * This file is a part of Tusky.
+ *
+ * This program is free software; you can redistribute it and/or modify it under the terms of the
+ * GNU General Public License as published by the Free Software Foundation; either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * Tusky is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
+ * the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with Tusky; if not,
+ * see <http://www.gnu.org/licenses>. */
+
+package com.keylesspalace.tusky;
+
+import android.app.IntentService;
+import android.content.Context;
+import android.content.Intent;
+import android.content.SharedPreferences;
+import android.preference.PreferenceManager;
+import android.text.Spanned;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+
+import com.keylesspalace.tusky.entity.Notification;
+import java.util.List;
+
+import java.io.IOException;
+
+import okhttp3.Interceptor;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import retrofit2.Call;
+import retrofit2.Callback;
+import retrofit2.Response;
+import retrofit2.Retrofit;
+import retrofit2.converter.gson.GsonConverterFactory;
+
+public class MessagingService extends IntentService {
+    public static final int NOTIFY_ID = 6; // This is an arbitrary number.
+
+    private MastodonAPI mastodonAPI;
+
+    public MessagingService() {
+        super("Tusky Pull Notification Service");
+    }
+
+    @Override
+    protected void onHandleIntent(Intent intent) {
+        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(
+                getApplicationContext());
+        boolean enabled = preferences.getBoolean("notificationsEnabled", true);
+        if (!enabled) {
+            return;
+        }
+
+        createMastodonApi();
+
+        mastodonAPI.notifications(null, null, null).enqueue(new Callback<List<Notification>>() {
+            @Override
+            public void onResponse(Call<List<Notification>> call,
+                                   Response<List<Notification>> response) {
+                if (response.isSuccessful()) {
+                    List<Notification> notificationList = response.body();
+                    for (Notification notification : notificationList) {
+                        NotificationMaker.make(MessagingService.this, NOTIFY_ID, notification);
+                    }
+                }
+            }
+
+            @Override
+            public void onFailure(Call<List<Notification>> call, Throwable t) {}
+        });
+    }
+
+    private void createMastodonApi() {
+        SharedPreferences preferences = getSharedPreferences(
+                getString(R.string.preferences_file_key), Context.MODE_PRIVATE);
+        final String domain = preferences.getString("domain", null);
+        final String accessToken = preferences.getString("accessToken", null);
+
+        OkHttpClient okHttpClient = OkHttpUtils.getCompatibleClientBuilder()
+                .addInterceptor(new Interceptor() {
+                    @Override
+                    public okhttp3.Response intercept(Chain chain) throws IOException {
+                        Request originalRequest = chain.request();
+
+                        Request.Builder builder = originalRequest.newBuilder()
+                                .header("Authorization", String.format("Bearer %s", accessToken));
+
+                        Request newRequest = builder.build();
+
+                        return chain.proceed(newRequest);
+                    }
+                })
+                .build();
+
+        Gson gson = new GsonBuilder()
+                .registerTypeAdapter(Spanned.class, new SpannedTypeAdapter())
+                .registerTypeAdapter(StringWithEmoji.class, new StringWithEmojiTypeAdapter())
+                .create();
+
+        Retrofit retrofit = new Retrofit.Builder()
+                .baseUrl("https://" + domain)
+                .client(okHttpClient)
+                .addConverterFactory(GsonConverterFactory.create(gson))
+                .build();
+
+        mastodonAPI = retrofit.create(MastodonAPI.class);
+    }
+}