Remove cmake and dummy.cpp in Android example

[beicause]

Objective

libc++_shared.so is no longer needed after #20323

Solution

Remove unused dummy.cpp and cmake in Android examples.

Testing

Run:

cargo ndk -P 26 -t arm64-v8a build -o examples/mobile/android_example/app/src/main/jniLibs -p bevy_mobile_example
cd examples/mobile/android_example
./gradlew assembleDebug
adb install -r ./app/build/outputs/apk/debug/app-debug.apk

[leomeinel]

This is awesome and works pretty well. Except the pending comments I approve. Similar changes have been tested as part of #23491 and it built and ran fine after reverting #23379.