Update getMobileProvisioningUrl to telephony surface

ConnectivityService is going to become mainline and can not
access hidden APIs. The mobile provisioning url is used by
telephony and it will use the hidden xml parsing APIs. It also
would make more sense to have the parsing inside telephony code.
Thus, move getMobileProvisioningUrl parsing to telephony surface
from ConnectivitySerivce and update the corresponding callers.

Bug: 175177794
Test: m and verify the url could be read successfully from the
      telephony surface
Change-Id: Ie8b46bc9b652f926277b10e36a86adba4dbd0121