SystemUI : Enable SettingsObserver for TransparencyManager

The transprency of statusbar can be set in Settings,
use the SettingsObserver in TransparencyManager to let the setting work.

Change-Id: I4f7b7211f513a83102d92170a509cdb008bdd6a3
Merge "SystemUI : Enable SettingsObserver for TransparencyManager" into jellybean