AudioService: rewrite sound effects loading

Handle all access to SoundPool in the message Handler to
synchronize load/unload and play requests.

Add a timeout to load operations in case a problem occurs while loading
a sample and the SoundPool callback is not called.

Bug 6633095

Change-Id: I39949e4c8b2f7ca2d8b760b1f92c7e0fcd0a46e1