Allow AudioTrack to start at nonzero position

When creating a new AudioTrack (not inheriting one from a previous play),
the AudioSink should take the AudioTrack's position as the initial starting
point for mBytesWritten, since otherwise NuPlayer's calculations will be off.
Normally this position will be 0, but if the test code for 32 bit wraparound
in AudioFlinger.cpp is enabled, it might be (much) larger.

Change-Id: I1e4f906d529861c3dea996de8afc6dbd491589af