Remove android game activity from default

Bevy previously had android-game-activity as part of its default features. Users that wanted to use android-native-activity instead, had to disable default-features and define all features plus android-native-activity explicitly.

Both options are no longer part of default-features, but they need to be added explicitly.

For apps using GameActivity you need to add the android-game-activity feature to your Cargo.toml:

bevy = { version = "0.19", features = ["android-game-activity"] }

For apps using NativeActivity you no longer have to define all features explicitly, you can simply use:

bevy = { version = "0.19", features = ["android-native-activity"] }