Choose fullscreen main window from task to create task snapshot window.

The insets attributes can be mismatch with snapshot if the top activity
is dialog, which make the the snapshot starting window shifting.
Core should also choose getTopFullscreenMainWindow as the attributes
of snapshot starting window, in case the top activity is translucent.

Flag: EXEMPT bugfix
Bug: 341020277
Test: launch app which will bring GrantPermissionsActivity on the task,
move task to back. Verify the snapshot starting window won't jump when
launch the task back to top.

Change-Id: I3c7b6b8952684f780117b971529cb079302dd1fa