QS: Link the screen timeout tile to the settings

This commit simply makes long-pressing the back of the sleep tile (the
screen timeout selection part of the tile) start the appropriate
settings activity (Display settings). This matches the behaviour of
most other QS tiles where there are additional settings screens.

Change-Id: Ia36764f1a606cd3166a11516fe3709ce9bffd125
Merge "QS: Link the screen timeout tile to the settings" into kitkat