Disable leak detection in URLConnectionTest by extending TestCase.

URLConnectionTest previously extended
AbstractResourceLeakageDetectorTestCase which supplied the
leak detection logic, but that logic was unreliable.

This created more boilerplate and maintenance work in unrelated
tests (which are not focused on leak detection) exceeding the
value it provides:

  - The leak detection doesn't work at all under CTS (29533469)
    and is very flaky on vogar.
  - Currently URLConnectionTest constructs 19 instances of
    TestSSLContext and 127 instances of Http(s)URLConnection,
    most of which aren't closed/disconnected. Closing all of
    these would add excessive boilerplate and pollute the
    purpose of these tests.

Tested:
 - Test now passes under vogar

Bug: 29533469
Change-Id: I5ef6a51e651b52d975a4cd0dfe4548b83a328c72