Status bar brightness control on lock screens

The status bar brightness control does not work on lock screens,
altough it would be useful when you try to type your password
or pin code – e.g. in bright conditions.

Patch Set 3:
- Remove the unnecessary "else" in "if" statement
- Use mViewDelta in brightnessControl method as is in the original code

Change-Id: Idf086f35d9952a892c932780be2192a26a4b38fa