Disabled Animations for Brightness Dialog

Brightness dialog enter animations now disabled when trigerring the
brightnessdialog with keyboard shortcut. BrightnessDialog Activity will
now be started with the FLAG_ACTIVITY_NO_ANIMATION flag to disable enter
animation  and exit animation is an instant fade out.

Test: Change brightness using keyboard shortcut -> ensure brightness
slider appears without animation -> wait for brightness slider to
disappear -> ensure brightness slider disappears without animation.
Fixes: 304970950
Flag: NA

Change-Id: I18307df735a87cd222780e83e9cc09df2c7a692d