libstagefright: Deregister audio flinger client during LPA session close

-Mediaserver crash is observed during BT disconnect.
-Audioflinger client registered during lpa playback is never
 deregistered.
-Fix this by deregistering audioflinger client during LPA
 session close.

CRs-Fixed: 407397
(cherry picked from commit d36aeb2d863a0ba42f45dbb59cd144dbf2c62cd6)

Change-Id: I19ef959a3d35476b4689e6c917a0cf6ccb4ffa78