Only fetch and display a given notification once (#3626)

When fetching:

- Maintain a marker with the position of the newest fetched notification
- Use the marker to determine which notifications to fetch
- Fetch notifications with min_id to ensure that none are lost
- Update the marker as necessary
- Perform a one-time immediate fetch of notifications on startup

When creating notifications:

- Identify each notification with tag=${MastodonNotificationId}, id=${account.id}
- Remove activeNotifications field, it's no longer necessary
- Use the tag/id tuple to reliably identify existing notifications and avoid creating duplicates
- Cancelling notifications for an account must iterate over all the notifications, and individually remove the notifications that exist for that account.
- Limit notifications to a maximum of 40 (excluding summary notifications)
- Remove notifications (oldest first) to get under this limit
- Rate limit notification creation to 1 per second, so the OS won't drop them

Adjust the summary notification:

- Ensure the summary notification and the child notifications have the same group key
- Dismiss the summary notification if there is only one child notification

NotificationClearBroadcastReceiver is no longer needed, so remove it, and the need for deletePendingIntent.

Fixes #3625, #3539

app/src/main/java/com/keylesspalace/tusky/db/AppDatabase.java

@@ -18,7 +18,9 @@ package com.keylesspalace.tusky.db;
 import androidx.annotation.NonNull;
 import androidx.room.AutoMigration;
 import androidx.room.Database;
+import androidx.room.DeleteColumn;
 import androidx.room.RoomDatabase;
+import androidx.room.migration.AutoMigrationSpec;
 import androidx.room.migration.Migration;
 import androidx.sqlite.db.SupportSQLiteDatabase;
 
@@ -39,9 +41,10 @@ import java.io.File;
         TimelineAccountEntity.class,
         ConversationEntity.class
     },
-    version = 49,
+    version = 50,
     autoMigrations = {
-        @AutoMigration(from = 48, to = 49)
+        @AutoMigration(from = 48, to = 49),
+        @AutoMigration(from = 49, to = 50, spec = AppDatabase.MIGRATION_49_50.class)
     }
 )
 public abstract class AppDatabase extends RoomDatabase {
@@ -665,4 +668,7 @@ public abstract class AppDatabase extends RoomDatabase {
             database.execSQL("ALTER TABLE `TimelineStatusEntity` ADD COLUMN `filtered` TEXT");
         }
     };
+
+    @DeleteColumn(tableName = "AccountEntity", columnName = "activeNotifications")
+    static class MIGRATION_49_50 implements AutoMigrationSpec { }
 }