Prevent BrightnessDialog to finishing twice in test

Calling finish in the real dialog will lead to a bad state in the
ActivityTestRule. This can lead to trying to finish after the test has
finished and cleared mocks.

Test: atest SystemUITests
Fixes: 309737682
Flag: None
Change-Id: Ie3152a889afe2c84b378540356cac115315ac16d
2 files changed