Backup device information

Adding the device information to the backup will allow us to restore
data which is device specific if it's appropriate to do so (e.g. device
density)

The device specific settings which will be backed up are;

Settings.Secure.DISPLAY_DENSITY_FORCED

Test: atest SettingsProviderTest SettingsBackupTest
Test: Manual - Density changes during SUW when setting is restored
Fixes: 28437818
Change-Id: Ibc3595cdece3f1ccb4fccaff8212c1c3cb5c9756