AudioFlinger: more fixes for offload audio lost

mStandby being false was not a reliable indication that
something had been written to audio HAL.

Considering the last track in mActiveTracks vector as the
latest activated track was wrong as this is a SortedVector.

Bug: 11247103.
Change-Id: I397d011c5afcdd779def3fe25aaae7669a472cfc