Remove android-activity dependency + add activity features (#2863)

Instead of depending on android-activity directly, this exposes the
android-native-activity and android-game-activity features from Winit.

This ensures that applications can choose what android-backend they use
while also relying on Winit to decide what version of android-activity to
use - without increasing the risk of a version conflict by having a direct
dependency.

_(NB: Egui doesn't currently use the android-activity API itself)_

Since android-activity provides the `android_main()` entry point for
Android applications it's not possible to link in multiple version of
the android-activity crate and so it's particularly important to
avoid unnecessary direct dependencies that could cause a version
conflict in the future.

To help avoid the need for applications to directly depend on
android-activity the Winit crate re-exports the android-activity API
and exposes features to configure the backend so that application crates
can instead rely on Winit to pull in a compatible version of
android-activity. (This way version bumps for android-activity only
need to be synchronized with the Winit crate).

CI now enables the `android-native-activity` feature for testing.

Fixes: #2829
Fixes: #2720
Closes: #2834

.github/workflows/rust.yml

@@ -205,7 +205,7 @@ jobs:
       - name: Set up cargo cache
         uses: Swatinem/rust-cache@v2
 
-      - run: cargo check --features wgpu --target aarch64-linux-android
+      - run: cargo check --features wgpu,android-native-activity --target aarch64-linux-android
         working-directory: crates/eframe
 
   # ---------------------------------------------------------------------------