Rename BouncerWindowBlurTestUtil to KeyguardWindowBlurTestUtil.

This changelist updates the name of BouncerWindowBlurTestUtil as it can
also be used to test other root window blurring, such as from glanceable
hub.

Test: related transition view model tests have been updated.
Flag: EXEMPT refactor
Bug: 389788272
Change-Id: I13674556474ff1563b0953d1ec7cb8d098d59598