ViewRootImpl: lazy initialization of mFastScrollSoundEffectsEnabled

mFastScrollSoundEffectsEnabled was initialized in the constructor
from a used-once instance of AudioManager.
Move initialization of this varialble to a lazy-initialization
where mAudioManager is also lazy-initialized, getAudioManager(),
which is only ever called in playSoundEffect(int), which reads
mFastScrollSoundEffectsEnabled.

Bug: 272459959
Bug: 266370934
Test: see steps in bug
Change-Id: Ib0bd59288197c05b13b9cb8a69d79960338c3c18