DevicePolicy: Restore backup manager if the device owner is cleared

Change I3fe54e43324c15dabd56834a923e8e8cb4eea98d permanently disables
the backup manager when a device owner is set. Put the service back
if the owner is cleared

Addresses CYNGNOS-779
Fixes android.security.cts.ServicePermissionsTest#testDumpProtected when
run after com.android.cts.devicepolicy.DeviceOwnerTest#testKeyManagement

Change-Id: I1c5bc39f6feb6be03975cedc16185378bbc28473