Better IME transition while switching app with recents (1/N)

Changes includes:
- In DC#computeImeTarget, keeps the IME target with the last window
  while swiping up to recents to prevent flickering due to IME hide
  animation on top of recents.
- Screenshot Task with IME window if IME is attatched to the app.
- Ignore hiding current IME if IME is attached to the app.

Bug: 166736352
Test: manual, test the app with focusing editor and show soft-input,
      swiping up recents and see if the task view with IME screenshot are
      shows up without flickering.

Change-Id: Ib898bc2e3a5b063fe9eaf8b07d022231dd1326a3