Add a screen to show OEM backup settings.

Moves the backup settings logic to BackupSettingsHelper and
BackupSettingsActivity:
- If the manufacturer provides the intent and the label for their backup
settings in the config.xml, a new intermediate fragment is shown for
Backup settings to let the user pick either standard backup settings or
OEM provided backup settings.
- If config.xml doesn't contain the intent, BackupSettingsActivity is
used as a trampoline activity to launch backup settings provided by the
backup transport of the default backup settings activity, i.e.
PrivacySettingsActivity.

Bug: 34700410
Bug: 33655074
Bug: 33654991
Test: make RunSettingsRoboTests
Change-Id: I78e340fbf926b2a9dc2c4e3942f9337c3c7a933c