Allow system services to get/set app-locales.

As of now, the LocaleManagerService uses enforceCallingPermission to
check for the required permissions before accessing / modifying the
app-locales.

Because of this, other system services are unable to pass
the permission check since they are running with the same pid as LMS.
We want to allow the backup service and settings to get / set app
locales.

enforceCallingOrSelfPermission allows apps / services with the same
pid as the system server to fetch / write app-locales. This is a
common practice among the system services.

Bug: 203649239
Test: Manually tested with the backup service
Change-Id: Iff96b2c8d97066c2521e923195eb02cf21200714