SoundPool: Clean up JNI weak reference handling

Remove unnecessary JNI params and overhead.
Use weak reference entirely on the native side.

Test: atest SoundPoolAacTest
Test: atest SoundPoolHapticTest
Test: atest SoundPoolMidiTest
Test: atest SoundPoolOggTest
Test: atest AudioManagerTest#testSoundEffects
Bug: 223815163
Change-Id: I31964b27ab3e8a96f49794e78537b165e9e84ed3