Re-compute IME target after the transition finished

This replaces previous approach (440f0683) with a simpler condition
at a more explicit place.

The old condition !isPlaying is always satisfied in normal traversal
that causes endless surface placement. And it might be better to
keep checkAppTransitionReady focus on legacy things, to avoid mixing
unnecessary operation while using shell transition.

Bug: 210390668
Test: 1. Enabling shell transition.
      2. Launch Settings and click search bar to show IME.
      3. Swipe from bottom to home.
      4. Click the search bar of app drawer, IME should show.
Change-Id: I89cdbcd19710720383f4488d859a7453a14c8c0f