Make isInvisible ActivityEmbeddingSecondaryActivity check optional

We don't always have an in between frame with the ActivityEmbeddingSecondaryActivity being invisible once the main activity doesn't cover the entire screen.

Test: atest com.android.server.wm.flicker.activityembedding.OpenTrampolineActivityTest

Bug: 288591571
Change-Id: I9846ef34a7d9e007031c85545c58676b78dcb60d