Ime target window should control when to hide IME (2/2)

This followup CL implements hideInsets() introduced in 1/2.

Bug: 142461756
Bug: 111084606
Test: Manually tested using steps below:
  1. Make sure new insets flag is enabled
  2. Launch any activity which has child window with NOT_FOCUSABLE,
     ALT_FOCUSABLE_IM (e.g. Instagram login screen)
  3. Verify IME can be shown and hidden by this window.

Change-Id: I307594014eca8a06397c739ffbc9c12eac160fdc