Fix AudioEngine to allow re-initialization

Add AudioEngine::CompareAndSetEngine and change
VariableSpeed::initializeEngine to use this method so it only initializes
a new engine once rather than asserting on subsequent calls.

Change AudioEngine::DeleteEngine so it ignores repeated calls instead of
asserting. Add VariableSpeed::isReadyToPlay so callers can detect when
the audio library has been properly initialized.

Bug:11511992
Change-Id: I4517cb2ab4c186da43dffc7dbc2a9ad49000059f