revert MainActivity launchMode again (#4685)

This fixes
- The problem where Tusky drops your draft when you switch apps while
composing
- The problem where MainActivity does not restart when switching theme
in preferences

This adds back a bug where one can have multiple instances of
MainActivity which can behave weirdly when the active account was
switched after they were created. This bug is (unlike the timeline mixup
one) transient though, it will go away when restarting the app. As a
small mitigation MainActivity is finished when forwarding to
ComposeActivity (Tusky 25 behavior).

app/src/main/AndroidManifest.xml

@@ -42,7 +42,6 @@
             android:name=".MainActivity"
             android:configChanges="orientation|screenSize|keyboardHidden|screenLayout|smallestScreenSize"
             android:exported="true"
-            android:launchMode="singleTask"
             android:theme="@style/SplashTheme">
 
             <intent-filter>

app/src/main/java/com/keylesspalace/tusky/MainActivity.kt

@@ -576,6 +576,7 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, MenuProvider {
             }
         }
         startActivity(composeIntent)
+        finish()
     }
 
     private fun setupDrawer(