Set communal view alpha.

This changelist alters the communal view's alpha during
transitions such as the shade expansion to be consistent
with the rest of the keyguard content, which dims out
during these events.

Bug: 197035411
Test: CommunalHostViewControllerTest#testAlphaPropagation
Test: NotificationPanelViewTest#testCommunalAlphaUpdate
Change-Id: I8c499d7c8746efe3b560935af09ff9da12c0e849