SystemUI: Allow showing of status bar when hidden and notification posted. [1/2]

When enabled, the status bar will be shown for a few seconds when a new
notification arrives and the status bar is hidden due to a full screen
app or expanded desktop being enabled.

Change-Id: Ifa18942ca14bf3b0dd8f3ce0c68187789d1c7a51
Merge "SystemUI: Allow showing of status bar when hidden and notification posted. [1/2]" into jellybean-mr2.1