Use and reparent the task's dim

Instead of using the display area's dimmer, each window has access only
to it's own task's dimmer. The associated dim layer can be reparented to
the display area when needed.

Bug: 352522056
Test: DimmerTests
Flag: com.android.window.flags.use_tasks_dim_only
Change-Id: I4723efbd6bd8fc3a6613970b244b1e69996a472a