Update splash screen to properly use the Androidx SplashScreen library (#4413)

The Androidx SplashScreen library is added as a dependency to the
project but isn't properly enabled in the current code. This pull
request configures the splash screen properly.

- Remove `SplashScreenActivity` which is not needed and use
`MainActivity` as main entry point to the application. `MainActivity`
inherits from `BaseActivity` which already detects if no account is
configured and redirects to `LoginActivity` if needed, just like
`SplashScreenActivity`.
- Initialize the SplashScreen library in `MainActivity.onCreate()`.
- Instead of letting the SplashScreen library set the final theme from
the `postSplashScreenTheme` attribute in SplashTheme, let `BaseActivity`
set it according to the user settings.
- When no account is available in `MainActivity.onCreate()`, keep the
splash screen shown until `LoginActivity` appears.
- Disable the slide-in animation when launching `LoginActivity` when no
account is available because the detection happens in `onCreate()` and
an Activity that finishes itself in `onCreate()` will not be drawn, so
the slide-in animation will not be visible either and only
`LoginActivity` will appear.
- Upgrade `core-splashscreen` to 1.2.0-alpha01 which contains a fix for
corrupted app theme on API 31+.

app/src/main/AndroidManifest.xml

@@ -21,22 +21,6 @@
         android:localeConfig="@xml/locales_config"
         android:enableOnBackInvokedCallback="true">
 
-        <activity
-            android:name=".SplashActivity"
-            android:theme="@style/SplashTheme"
-            android:exported="true">
-
-            <intent-filter>
-                <action android:name="android.intent.action.MAIN" />
-
-                <category android:name="android.intent.category.LAUNCHER" />
-            </intent-filter>
-
-            <meta-data
-                android:name="android.app.shortcuts"
-                android:resource="@xml/share_shortcuts" />
-
-        </activity>
         <activity
             android:name=".components.login.LoginActivity"
             android:windowSoftInputMode="adjustResize"
@@ -56,8 +40,14 @@
         <activity
             android:name=".MainActivity"
             android:configChanges="orientation|screenSize|keyboardHidden|screenLayout|smallestScreenSize"
-            android:exported="true">
+            android:exported="true"
+            android:theme="@style/SplashTheme">
 
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
             <intent-filter>
                 <action android:name="android.intent.action.SEND" />
 
@@ -101,6 +91,9 @@
                 <data android:mimeType="audio/*" />
             </intent-filter>
 
+            <meta-data
+                android:name="android.app.shortcuts"
+                android:resource="@xml/share_shortcuts" />
             <meta-data
                 android:name="android.service.chooser.chooser_target_service"
                 android:value="androidx.sharetarget.ChooserTargetServiceCompat" />