[wm] Make dim animation duration respect transition animation scale

Fixes test flakniess by allowing tests to disable dim animations. Blur
is set by the dim layer and blur radius is affected by alpha. Tests
validating blur has to ensure the dim animation is complete
before taking screenshots. This change allows the tests to disable the
dim animation by setting the scale to 0 for the duration of the test.

Test: atest android.server.wm.other.BlurTests#testNoBlurBehindWhenBlurDisabled --rerun-until-failure
Bug: 293532283

Change-Id: Ia3873eb5185c5b3d456448018ca121a95c645e55