Apply ValueAnimator scale factor immediately in WM.

Normally the ValueAnimator scale factor is applied the first
time a ViewRootImpl window session is created but that may
be too late for animators created by system services that
start early in the boot process.  So set the scale factor
immediately whenever the setting changes.

Also make ValueAnimator.getDurationScale() accessible (but @hide)
for custom animators that want to apply the same scale to
their animations.

Change-Id: I0f5a750ab5b014f63848445435d8dca86f2a7ada