Expose latency methods through AudioManager and AudioTrack.

There's a longer term plan to fix audio/video sync, but
this gets the Java level to parity with the native level,
and allows applications in Java to achieve sync in the
same way as the native media player. APIs are left as hidden
for now.

Change-Id: Iaf70baac1ffb50ef48e03355163158568fbd0fe9