Add an api to verify if ro.device_owner was set

Currently for OobConfig app we used DEVICE_PROVISIONED Global setting to
verify if device was provisioned. This setting can be modified using
adb. we just need to know if it was set atleast once. Added an api in
DPM to return whether system property "ro.device_owner" was set.

Bug: 29935702
Change-Id: I9a2b5217c0bc2cc11d68282e05a5450ea3f6cf21