Apply dim changes only at the end of traversal

Collect the changes on a dim layer and assign only the last requested to
a transaction
- Avoids issues with transaction ending up in the wrong order
- Allows smooth transactions between different alpha values
- Does not treat the entering animation as a special animation case

Test: DimmerTests
Bug: 281632483
Bug: 295291019

Change-Id: Ic3e6ce1f107bfc3ac925eb167b8bc89a5df478c8