Fix issue 3354692.

The problem is that SoundPool never calls the onLoadComplete() callback
when load() method fails because the file cannot be opened.
AudioService loadSoundEffects() was blocked waiting for a callback
that never came.
In this particular case, this happened on a specific build where sound
effects assets were not included.

The fix consists in waiting for the callback only if at least one sample is
accepted by SoundPool load().

Also moved the initial loading of sound effect samples from the boot completed
broadcast receiver to the audio service message handler to avoid waiting in the
main system server thread.

Change-Id: I902e5988f8b644a81d8e585c52eed1df4ca0ec05