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/java/com/keylesspalace/tusky/MainActivity.kt

@@ -46,6 +46,7 @@ import androidx.coordinatorlayout.widget.CoordinatorLayout
 import androidx.core.app.ActivityCompat
 import androidx.core.content.ContextCompat
 import androidx.core.content.pm.ShortcutManagerCompat
+import androidx.core.splashscreen.SplashScreen.Companion.installSplashScreen
 import androidx.core.view.MenuProvider
 import androidx.core.view.forEach
 import androidx.core.view.isVisible
@@ -203,10 +204,15 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, MenuProvider {
 
     @SuppressLint("RestrictedApi")
     override fun onCreate(savedInstanceState: Bundle?) {
+        val splashScreen = installSplashScreen()
         super.onCreate(savedInstanceState)
 
         val activeAccount = accountManager.activeAccount
-            ?: return // will be redirected to LoginActivity by BaseActivity
+        if (activeAccount == null) {
+            splashScreen.setKeepOnScreenCondition { true }
+            // will be redirected to LoginActivity by BaseActivity
+            return
+        }
 
         if (explodeAnimationWasRequested()) {
             overrideActivityTransitionCompat(