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.

CRs-Fixed: 578165
Change-Id: I8173845e1782b5a8b372a82c7d337299a61e0efe