Tidy up MockWebServer usage

Make sure that MockWebServer instances are shut down
correctly after tests.

Test: Ran the tests under CTS
Bug: 29820565
Bug: 29365528
(cherry picked from commit 9cc268daf1a03b745f5c161173816a3340beffa0)

Change-Id: If9b5a0681cd1e0633accbb4cd84feb10d863c0d7
4 files changed