Don't include IME in app screenshot while in multi-window mode

We don't screenshot the IME in multi-window mode because the frame
of the IME might not overlap with that of the app. E.g. IME target
app at the top in split-screen mode and the IME at the bottom
overlapping with the bottom app.

Bug: 28446505
Change-Id: Iae4f7eafa582779a4cf2aba3d83c4d9f76a4ccca