Refactor AudioTrack JNI stub

- Use new callback/buffer interface
- Manage callback interface object lifetime with sp<> and ref-counting
- Combine JNI storage into a ref-counted object, with lifetime tied
to AudioTrack.java

Test: atest AudioTrackTest
Bug: 199156212
Bug: 216175830
Change-Id: I0ff9c90fa19f9f38287d827eaebdf58fa02aec5b