Move dim bounds handling into dimmer

Assumptions on who owns dim and who is its parent changed. Logic should
have needed to be revised and it's easier to move everything inside
dimmer rather than having all the different containers handle it by
themselves (with the current assumptions).

Improve test coverage for activity embedding and freeform bounds
assumptions.

Bug: 366132859
Test: DimmerTests
Test: BlurTests
Flag: com.android.window.flags.use_tasks_dim_only
Change-Id: I0eb887f5f75ad7ba3a60fb61ec2996114fccac7e