Remove the test window after the test finishes.

The remaining window can affect the rest of the tests because it won't
handle input events. This can cause an ANR error and abort the test
process.

Flag: EXEMPT bugfix
Bug: 427837652
Test: atest NotificationTemplateViewWrapperTest
Change-Id: I78c5dee11815f4413e2768797b993ad8fba1e206