[flexiglass] Update AltBouncerToGoneTransitionViewModelTest

- Disable tests that don't need to run when flexi is enabled.
For example, when flexiglass is enabled, there are no consumers
of the lockscreenAlpha, so we don't need to test that.
- Create flexi-specific new test for the notificationAlpha
which is currently still used when flexi is enabled. In the test,
fake the scene transition in addition to the keyguard transition.

Test: atest AlternateBouncerToGoneTransitionViewModelTest
Bug: 409851925
Flag: EXEMPT test change only
Change-Id: I15025ceb0ce3e3a22bc63816267af08b42c9d7e9