[Multi-user] Change Backup Settings page to support multi-user.

With multi-user backup, backup will be available or secondary users and profiles too.

1. When there are profile users, this CL changes UI flow A to B
A. Settings Homepage -> System -> Google Backup Settings for primary user
to
B. Settings Homepage -> System -> Profile Select Dialog -> Google Backup Settings for chosen profile.
Change done as per https://g3doc.corp.google.com/company/teams/apps-android-settings/howto/ia.md?cl=head
2. Functionality remains the same when there are no profiles (BackupSettingsHelper, BackupSettingsContentProvider)
3.  Also, enabled Backup Settings for secondary users. (Previously, backup settings were only shown for
system user).(SettingsGateway)

Bug: 121198738

Test: 1) System -> Backup -> shows Profile Select Dialog -> Backup page for selected profile
2) On secondary user without profile, System -> Backup (with summary) -> Backup page for secondary user.

Change-Id: I6e21279978a5dfc6eca6f5a34bbfc15a34eac68b