Clear invocation count before verifying mirroring method

The count may be increased in setUp from
applySurfaceChangesTransaction before mocking
performSurfacePlacement.

Fix: 295866207
Test: ContentRecorderTests# \
      testDisplayContentUpdatesRecording_displayMirroring
Change-Id: Ia4ae93c57dfce30ea09802d4be159f1c472d37d9