Add precondition checks

* Add precondition checks to check the call authorization
  and the calling user.
* Add CallerIdentity object and helper method to create
  and return a CallerIdentity object.
* Add isDeviceOwner check which takes in a CallingIdentity
* Update setLocationEnabled to use new precondition check

Bug: 162825394
Test: atest com.android.server.devicepolicy.DevicePolicyManagerTest
      atest com.android.cts.devicepolicy.DeviceOwnerTest#testSetLocationEnabled
Change-Id: I6054620aab661f302ebe0cb8c13bf906aad3cb68