audioflinger: Do not report underruns for stopped or paused tracks

PlaybackThread::Track is missing checks for the track state
when reporting underruns. These checks present on other instances
when tallyUnderrunFrames is being called.

This eliminates fake underrun reportings when playing
multichannel tracks via deep buffer.

Bug: 36181621
Test: play multichannel track and check underrun count at the end
Change-Id: Ieff338b33807e66df91234d85b782ee383fe95b9