Created new public API so applications can customize its metered network

usage while running in background.

The new API is 'int getRestrictBackgroundStatus()' and returns the
following values:

 - RESTRICT_BACKGROUND_STATUS_DISABLED: no restrictions
 - RESTRICT_BACKGROUND_STATUS_WHITELISTED: restriction but app is
   whitelisted
 - RESTRICT_BACKGROUND_STATUS_ENABLED: full restriction

The proper way to interprete these values for using metered networks
while running on background is:

 - When disabled, there is no restriction and the application could use the
   metered networks freely.

 - When whitelisted, the application can use the metered network, but should try
   to minimize the usage.

 - When enabled, the application should not try to use metered networks at
   all, since the usage will be denied.

BUG: 26451391
Change-Id: If07d42bb88e4c02802df0234861f38aef2cfead7