Allow the user to change the BackupTransport

Set the following config overlays to activate this feature:

* config_backup_settings_intent to settings://com.android.settings.backup.transport
* config_backup_settings_label to some user-facing label
  e.g. Change backup provider

Squashed with:

Author: Michael Bestas <mkbestas@lineageos.org>
Date:   Wed Nov 18 16:08:44 2020 +0200

    Backup: Don't hardcode ignored backup transports

    * Allow extending the ignored backup transport list
      by moving it to an overlay
    * Set config_ignored_backup_transports to override

    Change-Id: If4ccad0acac708cac272f791392f9ac827a7491f

Change-Id: I080d96e2c34045a0e61f3fa1b839f463550f2028