audioflinger: fix lost offload thread resume event

It was possible that a resume request signaled by addTrack_l()
while waiting for an async write callback is lost. This is because
mSignalPending was not set and waitingAsyncCallback_l() would pause the
thread loop before executing prepareTracks_l().

The fix consists in using signal_l() method to wake the thread
loop o that mSignalPending is set.

Also make sure that sleepTime is reset to 0 when resuming to make sure
that we write any remaining bytes to the HAL.

Bug: 10810347.

Change-Id: If9a3b22cc3b9e6eb384a56c48c40e6258d0896ad