Prevent some leaks from leaking to the next test

Now when exiting the test, we finish Launcher activity and force the leak check.
I've seen this helping one of the leaks to stay within the test that caused it instead of failing the next test.

Bug: 187761685
Flag: N/A
Test: presubmit, local
Change-Id: I7eac26f749433c37a6880a2f035ab1ba9c5fef6e