fix & chore: unable to build release apk because of aggressive class keeping rules

See https://stackoverflow.com/a/76806805 for details.

Author: w568w

android/app/proguard-rules.pro

@@ -1,9 +1,6 @@
-#Flutter Wrapper
--keep class io.flutter.app.** { *; }
+# Flutter Wrapper
 -keep class io.flutter.plugin.**  { *; }
 -keep class io.flutter.util.**  { *; }
 -keep class io.flutter.view.**  { *; }
--keep class io.flutter.**  { *; }
 -keep class io.flutter.plugins.**  { *; }
--keep class de.prosiebensat1digital.** { *; }
--dontwarn io.flutter.embedding.**
+-keep class de.prosiebensat1digital.** { *; }