Remove Udfps's BackgroundViewModel

There's no need to unnecessarily create a new
object for every alpha change. Instead, the view binders
directly will listen for alpha and color changes in separate flows.

Test: builds
Flag: ACONFIG com.android.systemui.device_entry_udfps_refactor DEVELOPMENT
Bug: 305234447
Change-Id: I7825436baed323418830a26a9855845eaf634ac4
4 files changed