Prepare to remove getDimmer

Since these changes:
I9d816c213d0cf7a7adc32ebbbf843e35ea734d47
I4723efbd6bd8fc3a6613970b244b1e69996a472a
I0eb887f5f75ad7ba3a60fb61ec2996114fccac7e

Activity windows always use the dimmer of Task.
Non-activity windows always use the dimmer of RootDisplayArea.

Bug: 435301249
Flag: com.android.window.flags.remove_get_dimmer
Test: TaskFragmentTest#testGetDimBounds
Change-Id: I92f56a8d177f9478f96aef46265aacfc60b7a8fa