Tentative fix for issue 3362362.

The problem is likely that one method is called on the AudioPolicyManagerBase
instance while it is still being constructed by AudioPolicyService.

To avoid this, the AudioPolicyService mutex is held by the constructor until the
platform specific AudioPolicyManager is constructed and the member
mpPolicyManager initialized.

Also added an initCheck() method to AudioPolicyInterface to verify successful
initialization of AudioPolicyManager.

A similar change is done in AudioFlinger constructor.
Also added some missing protections in AudioFlinger methods where the
playback thread list is parsed.

Change-Id: I006b244ec057e1bb0aa5ebe426ef006e3b171056