Tear down VD in test to ensure it doesn't interfere with other tests

If the VD is not torn down, it can still be executing code using the
same StubTransaction. The tests assert things on the StubTransaction so
other VD running on the same StubTransaction can interfere with the
test.

Test: ContentRecorderTests
Fixes: 242115683
Change-Id: I353659c17e9b84661d6437144b980ab3aaacae3f