Settings app Data Usage UI enhancements

Modified Settings app to support following new features in
Data Usage Summary screen:
1) Added two items to overflow menu
   a) Enable/disable data usage alerts: to enable/disable global
      switch for data alerts. Shown only when DataUsage provider
      is installed.
   b) Reset data statistics: to reset data stats for current tab

2) Added a new per app switch "Mobile Data Alert"
3) Added a new per app switch "Restrict cellular data access"
4) Modified per app background data access feature to support
   three options
   a) Over mobile and WiFi
   b) Only over WiFi
   c) No access

DataUsage provider, service and notification receivers have been
refactored into a standalone package to minimize the changes to the
Settings App.

Change-Id: Ic0e42e57d04dc8e164ebfdab4531cced31c4e6eb
Issue-Id: SETTINGS-75
Issue-Id: SETTINGS-76
Issue-Id: SETTINGS-77
Issue-Id: SETTINGS-76