Add Bevy GameActivity Android runtime

[enfipy]

Objective

Fixes #139. Add first-class, Bevy-first GameActivity packaging and lifecycle support on Android.

Solution

• Add the game-activity runtime, AGDK/AppCompat dependencies, manifest generation, and resolved Bevy feature validation.
• Replace the deprecated Fragment bridge with a small Activity-owned plugin controller.
• Add a Bevy example, documentation, tests, and CI APK verification.

Validation

Built NativeActivity and GameActivity APKs, compiled the Android library and plugins, and passed Rust tests, Clippy, formatting, Android lint, and toolchain consistency checks.

Changelog

Added Bevy GameActivity support and removed obsolete Android Fragment/JNI lifecycle code.

Migration Guide

Custom Android hosts using CrossbowHost should construct Crossbow(this) after super.onCreate and attach crossbow.view; the unused setup/main-loop callbacks were removed.