Transfer focus from the current focused window for two-finger scroll

When doing the two-finger scroll, we resize based on the main window.
However, the main window might not have focus (eg. diaglog displayed
on top). We need to grab focus from the window that actually has focus.

bug: 26604036
Change-Id: I05d0ed95c59901c925e242d4b9a856d1ca0a55f3