Correct illegal mockito usage.

According to the mockito FAQ use of thenReturn(mock(...)) is
forbidden. This fixes that pattern in the setup for
ZenModeFilteringTest.

https://github.com/mockito/mockito/wiki/FAQ#can-i-thenreturn-an-inlined-mock-

Test: runtest systemui-notification
Bug: 119063276
Change-Id: Ic62b18063e5837240255e1dd0f5e0feb450f22cb