Do not remove the snapshot starting window immediately...

...after the IME reports that it has finished drawing.
The IME could draw before the application window gains focus after the
device is unlocked. Therefore, treat the IME window's "drawn" state as
a condition for removal, rather than directly removing the snapshot
starting window.

Bug: 429531321
Flag: EXEMPT bugfix
Test: Verify that, following a screen off/on cycle and device unlock,
the snapshot starting window should not disappear immediately when the
IME is drawn, but rather wait for the app window drawn.

Change-Id: Idb730c8471518b51bf8c1c80a013954eceae7857