Add AsUser versions of existing DevicePolicyManager methods

Some API methods on DevicePolicyManager call underlying IDevicePolicyManager methods
that can be parameterised with a user id. DPM however only exposed methods for the current user.
This CL makes it possible to call them with any user id by introducing hidden methods on
DevicePolicyManager.

Bug: 15558397
Change-Id: I1786a64fa7cf5f6590ddd04cbf4527bb159f8795