Disabled brightness dialog when shade is open

We don't want brightness dialog showing up under notification shade
when brightness is controlled using keyboard shortcut. So now if
notification shade is open when the brightness dialog activity is
created, the activity will finish and the brightness dialog will go
away.

Test: open notification shade -> control screen brightness using
keyboard shortcut -> close notification shade immediately after ->
ensure that brightness slider did not appear on the home screen.
Fixes: 303231981
Flag: NA

Change-Id: I31ed8bd1dc73a1170e6cfc4f61cbd6b9bfe54965