fix(droidplug): Resolve Adapter through LoaderContext in init

[LucaCappelletti94]

init looked up Adapter with plain FindClass, while every other class went through LoaderContext. On a thread attached from native code, Android's FindClass searches only the system class loader (JNI tips), so init failed with ClassNotFoundException there even when the thread's context class loader was the application's. Dioxus's manganis (0.7.4 and later) gets the JavaVM from ndk_context and attaches the calling thread itself (activity.rs).

#311 hit the same system class loader failure for FutureException.

Adapter now resolves through the same LoaderContext, which tries the context class loader before FindClass. The Android test app now calls init from a thread it attaches itself, which covers this path in place of the Java-thread one, since init runs once per process.

On a Galaxy M52 (Android 13) and two Galaxy A35s (Android 14 and 15), the test app's setup fails with failed to resolve Java class 'com/nonpolynomial/btleplug/android/impl/Adapter' before this change, and testAdapterAddress passes after it.

[qdot]

If this ends up fixing my Galaxy issues on my other software, I owe you greatly. :)

[LucaCappelletti94]

Lmk!