Add summary for Backup menu in Settings.

'On'/'Off' depending on whether the backup is enabled in settings.

Refactors BackupSettingsActivity to be shown on System fragment via
xml/PreferenceController instead of category in the manifest.

Bug: 36891827
Test: make RunSettingsRoboTests
Change-Id: I61425fed4d63055c6c386be38792ae074aa72679
5 files changed