Enable IME reparenting onto app window.

In Id40470f6f8284b48acfa4719049afd14fde332d6 we selectively reparent IME
to currently focused app window (if it is a fullscreen app).
This CL enables it for Q.

This is already a decent improvement in terms of transitions:
1. IME will no longer animate-out at a different speed outside the app
when Home button is pressed. IME will animate-out within the app window
making it look better.
2. Looks way nicer with quickstep UP gesture and consistent with upcoming gestural
navigation changes.

No logical behavior change for App or IME developers. Only IME
transitions are improved.

Bug: 37095064
Bug: 111084606

Test: Manually tested as follow:
  1. Build & flash
  2. Launch an app with textView e.g. Messages.
  3. Focus the textView.
  4. Swipe up slowly and observe IME transitions down within app window.
  5. relaunch app and focus textView
  6. Press home button and observe IME transitions out with app window.
  7. Repeat steps 2 - 6 for adjustResize apps like Whatsapp and adjustPan
     like play store, gmail.
  8. Use EditTextVariants and verified SOFT_INPUT_STATE_ALWAYS_VISIBLE
     works as intended while switching between apps.
  9. put in split-screen mode along with another app with textView like
     gmail.
  10. Swiped up and observed IME window animates outside the app window
      (no transition change)
  11. Again, in messages, while textView is focused, put Gboard into
      floating mode.
  12. Swiped up and observed IME transition is outside app window
      (unchanged)


Change-Id: I2065d82372316bf2a173cdfd9c92ee3673f6bd4d