AudioTrack: add setBufferSizeInFrames()

Also add getBufferCapacityInFrames().
These can be used to dynamically raise or lower latency.
Fix JavaDocs.

Bug: 21019153
Change-Id: I5c795c23826f7f29038d0de7004f4b7f365cd67e
Signed-off-by: Phil Burk <philburk@google.com>