Fix Quick Settings for MSim [1/2]

 - Show the data usage of active data SIM for QS
 - Immediately update QS when data subscription changes
 - Forward setDataEnabled AS-IS (see part [2/2])

Change-Id: I1c72d9955b1d068ef276386b761f7aa2d664fdc2
SystemUI: status bar brightness: store value as int

Non-automatic brightness value is stored as int, not float.

Symptom: adjust the brightness in the status bar, the brightness
         slider in the notification bar will always be set to full

Change-Id: I5f9a6e97bae2c44c3aff7c9222186a6dbda5d902