Added Backup & Restore for SystemUI and controls

This CL adds support for Backup & Restore in SystemUI by adding
Key-Value BR through the use of a BackupAgent (BackupHelper.kt). Right
now, the only thing that is backed up in SystemUI is the
controls_favorites.xml file.

BackupAgent will send a broadcast to user 0 indicating that the restore
has been completed for a particular user.

Regarding controls BR:
* The file is set as changed every time it is written and will
participate in normal backups as determined by BackupManager.
* On restore, a copy is made to serve as cache of controls restored but
whose app is not installed yet.
* This cache is persisted for a week and deleted with a JobService.

Test: atest SystemUITests
Test: manual using LocalTransport
Fixes: 149210531

Change-Id: I6048f1ebd1d9021058778dff707c8c53c9989b35