Hook up data saver to battery saver.

1. Add globalBatterySaverEnabled in PowerSaveState, which stores
the real battery saver mode, not the one for specific service.
2. Add "mRestrictBackgroundChangedInBsm" and
"mRestrictBackgroundBeforeBsm" to restore the mRestrictBackground
if necessary.
3. If user toggles the data saver when battery saver is on, don't
restore the data saver when battery saver is off.

Bug: 34693888
Test: FrameworksServicesTests
Change-Id: Ic50c1cae3fb110a228e59e5d1fd04c164e4b7274