Optimize the alpha reset of notification content views

Previously, we reset the alpha of all Content Views (private layout,
public layout, and children container) to 1 when the showing layout
changes to avoid accidentally make a transparent content view visible.
However, this introduces a issue: when an alpha animation is going on a
view and its alpha is reset, there will be a flicker. This change fixed
this issue by setting the old alpha back to the new showing layout
after the alpha reset to avoid breaking the animation.

Bug: 292024656
Test: atest ExpandableNotificationRowTest
Flag: ACONFIG notification_content_alpha_optimization DEVELOPMENT
Change-Id: I68368ecf93cd654ed6ebe74a49efc2ea2e012a8b