Fade keyguard status bar during transition to and from Glanceable Hub.

This change ties the keyguard status bar alpha to the transition
progress to and from Glanceable Hub.

Fixes: 374017195
Test: atest KeyguardStatusBarViewControllerTest#animateToGlanceableHub_affectsAlpha
Test: atest KeyguardStatusBarViewControllerTest#animateToGlanceableHub_alphaResetOnCommunalNotShowing
Flag: EXEMPT bugfix
Change-Id: Ib24526c8d57bfc5e689a3450402cb7d28f64f109