Make device entry icon alpha a SharedFlow (hot flow)

Previously, it was possible for the device entry icon
to receive old alpha values from a cancelled transition.
For example: LOCKSCREEN => AOD => LOCKSCREEN

Test: enable aod, press power key to screen off/on
multiple times. observe device entry icon is the right alpha
Fixes: 329386658
Flag: ACONFIG com.android.systemui.device_entry_udfps_refactor TEAMFOOD

Change-Id: I2c8b1dfd1619a043c1689f07decddf9c9e96d58c