Avoid changing resize dim layer if not needed

UPDATE_DOCKED_STACK_DIVIDER gets called fairly frequently.
When we switched to having a stack per task,
TaskStack.resetAdjustedForIme was called a lot more frequently,
leading to a lot of empty surface transaction binder calls, which
took very long to execute.

We fix this by avoid calling setResizeDimLayer as well as avoid
even creating a transaction when the dim layer hasn't changed.

Test: go/wm-smoke
Test: Resize docked stack, check IME adjust animations
Change-Id: I75e64c6e3ac77012388c68c6750a426bd3ed6e21
Fixes: 67747409
Bug: 67780926