audioflinger: improve record start and thread exit

Use broadcast() instead of signal() on the
thread wake up condition when starting record or requesting thread
exit to make sure that if another thread is waiting for the same
condition (e.g binder thread calling setParameters()) the mixer
thread will be woken up.

Bug 7184317.

Change-Id: I3154a4509ca7af6ffae5236e522b0fab8e75ed06