AudioTrack: get/setStartThresholdInFrames

setStartThresholdInFrames is used to set the start threshold
in frames for streaming AudioTrack playback.
Normally this is the entire buffer capacity in frames
but may be reduced for low latency playback,
compressed formats, and direct tracks.

Test: atest AudioTrackTest#testStartThresholdInFrames
Test: atest AudioTrackTest#testStartThresholdInFramesExceptions
Bug: 183003720
Merged-In: Iae10764130160de2380477134df27c5b139468bc
Change-Id: Iae10764130160de2380477134df27c5b139468bc