Update PiP Flicker tests for content overlay

For some reason, the app icon overlay which is backed by hardware buffer
does not occlude the pipApp in the trace, though both the screen
recording and winscope trace suggest so.

Per discussion, we update the assertion that
- pipApp layer is always visible
- content overlay appears and then disappears
during the transition.

Bug: 265998256
Test: atest WMShellFlickerTests:EnterPipOnUserLeaveHintTest
Change-Id: I402aa1baf5c829fb12c30af659bc2827d65853a0