audioflinger: offload: fix pause/flush/resume

If a pause/flush/resume sequence is fast enough, resume is received while
we are still in PAUSING state in which case it is a NOP. If this happens,
flush is still forwarded to the audio HAL but is not preceeded by a pause
which can cause problems to the audio DSP.
It is necessary to preserve the flush as this sequence is typical to a seek.

The fix consists in forcing a pause/resume when a flush request must be
executed and the audio HAL has not been paused previously.

Bug: 11081559.
Change-Id: Ib84ed26d503a61c05933b923ec556b10cedfe140