services: devicepolicy: Restore backup service as system

Restoring the backup service must be performed by either system
or root otherwise it will cause a SecurityException. This patch
clears the uid, performs the call as system and then restores it.

Fixes the following CTS tests:

com.android.cts.devicepolicy.CustomDeviceOwnerTest
  -- testOwnerChangedBroadcast
  -- testSilentPackageInstall

com.android.cts.devicepolicy.DeviceOwnerTest
  -- testCaCertManagement
  -- testCannotSetDeviceOwnerAgain
  -- testDeviceOwnerSetup
  -- testKeyManagement
  -- testLockTask
  -- testSystemUpdatePolicy
  -- testWifiConfigLockdown

com.android.cts.devicepolicy.MixedDeviceOwnerTest
  -- testAccountManagement
  -- testApplicationRestrictions
  -- testDelegatedCertInstaller
  -- testPermissionGrant
  -- testPermissionGrantPreMApp
  -- testPermissionMixedPolicies
  -- testPermissionPolicy
  -- testPermissionPrompts
  -- testPersistentIntentResolving
  -- testScreenCaptureDisabled

Change-Id: Ia26222f1fece14217554459c42679f82f7ceb980