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.

See CTS test AudioTrackTest#testStartThresholdInFrames
for example calling details and behavior.

Test: atest AudioTrackTest#testStartThresholdInFrames
Test: atest AudioTrackTest#testStartThresholdInFramesExceptions
Bug: 183003720
Change-Id: I5064d04961e48b530c49071ff84c2e0d2065f41b