http://ag/12885739 introduced a enforceCallingPermission(BACKUP) check
but callers of this API do not hold that permission. This CL fixes
this by changing the check to enforceCallingOrSelfPermission(BACKUP),
and clearing the binder identity in DevicePolicyManagerService, which
makes the system server process the owner of that call.
Bug: 158482162
Bug: 172466964
Test: atest com.android.cts.devicepolicy.{Device,Profile}OwnerTest#testBackupServiceEnabling
Change-Id: I11d863229c4d62a058aaf37446a694b9c73ae5b8