Fixed flickering when quick tap on brightness bar

Reduce the height of brightness mirror view.

Change-Id: I998932dea51659cbba0a8484973b3281f595c46e
Fixes: 79927315
Test: quick tap on brightness bar and observe the result
Test: atest google/perf/jank/SystemUI/UbSystemUIJankTests-Trace:android.platform.systemui.tests.jank.SystemUiJankTests#testChangeBrightness