Make user-switch transitions customizable

Make UserSwitcherController subclassable

Add ability for keyguard to animate in without a screen freeze
by adding a keyguardAnimatingIn state that WindowAnimator and
WallpaperController can use to prevent graphical glitches

Bug: 29329555
Change-Id: Idfd40156538b4bfb1777ad1a9a566e7da54e8f32