Improve wait for PIP window condition on app helper

PIP window displays a dismiss button (X) upon start. To ensure the animation is complete wait until the dismiss button disappear to continue the test. This mitigates flakiness (e.g., test finishes before focus change)

Bug: 196213706
Test: atest WMShellFlickerTests:com.android.wm.shell.flicker.pip
Change-Id: Ib0d3d4fee46d44fa0480f7e91f8c584c2d7613c9