Fix underruns when sample rate != native sample rate

This forces a minimum of 3 application buffers when the sample rates
don't match, using the normal mixer and low latency HAL.

There is still an issue that the latency() varies depending on whether
screen was off or on at the time of creating the AudioTrack.

With screen on:
I/AudioTrack( 2028): afFrameCount=960, minBufCount=2, afSampleRate=48000, afLatency=50
I/AudioTrack( 2028): minFrameCount: 2646, afFrameCount=960, minBufCount=3, sampleRate=44100, afSampleRate=48000, afLatency=50

With screen off:
I/AudioTrack( 2817): afFrameCount=960, minBufCount=4, afSampleRate=48000, afLatency=84
I/AudioTrack( 2817): minFrameCount: 3528, afFrameCount=960, minBufCount=4, sampleRate=44100, afSampleRate=48000, afLatency=84

Change-Id: Ib45515edff2afcd672dda34881b658c800ffc25a