Add Android RX888 MkII support

[wdarney]

Summary

Adds the Android RX888 MkII source-module path and supporting Android runtime plumbing needed to stream from the RX888 on SDR++ Android builds.

This PR also captures the Android audio and Channel Bank adjustments made during device testing so the branch is usable as the current working baseline in this repo.

What Changed

• Enabled RX888 source builds in the Android Gradle/CMake configuration.
• Added Android USB permission plumbing and packaged the FX3 firmware image.
• Added an Android-specific RX888 module entrypoint backed by the imported SDDC/RX888 core.
• Added Android libusb/FFTW/NEON build support for the RX888 core.
• Improved the Android audio sink with callback buffering, direct monitor playback for Channel Bank, and muted-stream sleep behavior.
• Enabled Channel Bank Android build/playback behavior without Whisper.
• Added detailed porting notes in docs/RX888_ANDROID_PORTING_NOTES.md.

Validation

• Built Android release APK with:
  JAVA_HOME=/opt/homebrew/Cellar/openjdk@11/11.0.30/libexec/openjdk.jdk/Contents/Home ANDROID_SDK_ROOT=$HOME/Library/Android/sdk ANDROID_HOME=$HOME/Library/Android/sdk /Users/willdarney/.gradle-versions/gradle-6.8.3/bin/gradle assembleRelease
• Installed successfully to Samsung test phone over ADB.
• RX888 MkII firmware permission flow was exercised on-device.
• RX888 streaming produced live waterfall on Android.
• Channel Bank monitor playback was audible after Android audio sink changes.
• User testing showed source/audio debug stats materially affected audio stability, so normal UI telemetry was removed from the final baseline.

Notes

This PR is intended for wdarney/SDRPlusPlus only. It is not cleaned up as an upstream SDR++ submission.

Be sure to not compile the app with debug on, lower end devices will suffer.