Proper fix for setDeviceOwner() permission check.

Previous fix was clearing the binding identity for the whole
manageUserUnchecked() call, which would break setting it using adb -
this change limits the scope just to the package manager calls.

Test: atest com.android.bedstead.nene.devicepolicy.DevicePolicyTest#setDeviceOwner_deviceOwnerIsAlreadySet_throwsException
Test: adb shell dpm set-device-owner --user 0 com.afwsamples.testdpc/.DeviceAdminReceiver

Fixes: 200810234

BYPASS_INCLUSIVE_LANGUAGE_REASON=existing API

Change-Id: I82e4ea2bef710420306ab17f58255ef7bc9cbb06