AudioTrack: relax check on minimum buffer size

Current AudioTrack implementation enforces that the requested audio
buffer size is at least corresponding the audio latency.
This requirement is too strong and leads to problems with current
stagefright and AudioSink implementations when playing over output
streams with long latency.

Ultimately, the AudioSink design should be changed to specify a minimum
buffer size in time or frames units but not in buffer count units.

Change-Id: I8ba603956f92ac49143a8249572665aa548f2f0f