AudioService: add scheduleLoadSoundEffects

Instead of instantly loading sound effects during onSystemReady(), move
it to a protected method such that sub-classes can override it and
schedule to load it at a later time.

Bug: 129433717
Test: Build Taimen, sound effect still comes in at boot
Change-Id: Ic0e4e963df23392fc9d5dd1b1fea9a62020530ac