audioflinger: fix record thread exit pending check

RecordThread loop must not release the mutex after checking for
exitPending and before waiting for a new wake up condition.
This can happen under the hood when methods like processConfigEvents_l()
or checkForNewParameters_l() are called. So exitPending() must
be checked after calling these functions.

Bug: 12787961.
Change-Id: Ia18c518bd5344fbb2401067303fcfe76a86879c4