Use ConnectivityManager.MockHelpers

Telephony tests mock ConnectivityManager private methods
but don't have access to their private types. In the
context that Connectivity would like to change the return
type of CM#registerNetworkAgent to a private type, offer
a simple solution to still mock it without having to
remove all checks from Telephony.

See also aosp/3543704

Test: FrameworksTelephonyTests
Change-Id: I61e5fa4b8ba6d8825d2c41bae907a0492c1f8b14
Flag: TEST_ONLY