Apply contentAlpha for brightness mirror to notifications

The alpha value is:
* Stored in NotificationPlaceHolderRepository
* Bound to NotificationStackView
* Combined (via min) with other alphas in NSSLController
* Applied to NSSL

Fixes: 332558331
Test: atest SystemUiRoboTest
Flag: com.android.systemui.scene_container
Change-Id: I6f88117d767b9a8e7eb76c8f27089f0b532ada9c