Create getMobileProvisioningUrl in telephony

ConnectivityService is going to become mainline and can not
access hidden APIs. Telephony and Settings were both accessing
the hidden API ConnectivityManager#getMobileProvisioningUrl.
Moving #getMobileProvisioningUrl method into telephony means
that there is one less access to a hidden API within the
overall framework since the connectivity stack never needed
this value. Thus, add getMobileProvisioningUrl() to telephony
surface to replace the one in connectivity stack.

Bug: 175177794
Test: atest FrameworksTelephonyTests
Change-Id: Icff8bf39b168b28f972b6737e4e7be8fc86b3dee