Reapply "Return the registry in registerNetworkAgent."

Now that faulty Telephony tests have been disabled temporarily
to allow for this bugfix, this can be reverted.
This reverts commit 653d5f1ef42cef52203c06d40027096a70e381f6.

Test: FrameworksTelephonyTests CtsNetTestCases
Change-Id: If6b1541981803dc84de57b48adb13d40c9997fa0
9 files changed