Don't give the IME child window focus when switching tasks

When switching the app task, we keep the IME window visibility for
better transitioning experiences.

However, in case IME created a child window without dismissing
during the task switching to keep the window focus because the IME
window has higher window hierarchy.

Hence, we don't give it focus if the next IME layering target
doesn't actually request the IME visible.

Fix: 186329504
Test: manual as issue steps:
  1) Tap any text box to bring up Gboard.
  2) Tap access point -> '..' -> Share Gboard or Quality Bug Report.
  3) Swipe up to Home screen.
  4) Tap another text box.
  6) Verify if the IME can shown as expected after editor focused
Test: atest DisplayContentTests#\
      testImeChildWindowFocusWhenImeLayeringTargetChanges
Change-Id: I72618745542f33bcbb1001bd3b812e79f7652dda