audio policy: fix lockup during mediaserver restart

When pre or post processing effects are applied by audio policy service
automatically there is a potential lockup if the first call into
AudioPolicyService is made while creating one of those effects.
This is because effects are created with AudioPolicyService mutex held and
effect creation calls into methods (e.g registerClient()) which also acquire
the mutex.

The fix consists in adding a new mutex to AudioPolicyEffects class and not
hold the AudioPolicyService mutex when calling methods in of class.

Bug: 17830596.
Change-Id: Ie61c3671d3147f46e99ba208f39c1a3bf180779f