Fix ScrimControllerTest memory leak

Change I577e352eda47af4a03ffdff2aaa788f6f853238c started attaching the
scrim view to collect flows for testing, which didn't happen before.
However the view was never detached so the flows never stopped
collecting and thus each test class instance was never GCed.

This change detaches the view to stop flow collection. See bug for
before and after hprofs. Also fixes the same issue in
GlanceableHubContainerControllerTest.

Bug: 322191262
Fixed: 322191262
Test: atest ScrimControllerTest GlanceableHubContainerControllerTest
Flag: N/A
Change-Id: I155c38f72d33db82bdd185ba366b11711a01236c