audioflinger: OffloadThread state fix

When switching from one offload session to another that
reuses the same offload output, audio HAL is paused & flushed in
preparation for the new track

OffloadThread must reset mHwPaused to allow proper state transitions
for the second track. In addition, Offload thread need not
explicitly call resume on HAL while handling a flush.

Bug: 12530661
Change-Id: I8173845e1782b5a8b372a82c7d337299a61e0efe