Don't remove starting window for visible app.

Activity manager agressively removes starting windows to avoid leaks.
See ag/696830 and ag/1213075 for some history. However, it is possible
for window manager to process the request of removing the starting window
coming from activity manager while the app token for the starting window
is still visible from the window manager side because we are delaying
hiding it due to an animation in progress.
We now only process the request to remove starting window from the
activity manager side if the app is hidden on the window manager side.

Change-Id: Ia33076f0e7405f9430e9d4e9d4026ae6fd1b133e
Fixes: 62443388
Test: go/wm-smoke
Test: Swipe Gmail away in Recents, launch Chome, press home button, launch Gmail, and press the home button.