Better IME transition while switching app with recents (4/N)

This CL introduces TaskSnapshotController#snapshotImeFromAttachedTask to
attach IME screenshot when performing closing transition.

This can improve app transition without jank or flickering while the IME
insets control transits from closing task to the next task, we keep the
IME surface visibility by placing the IME screenshot with calling
DC#showImeScreenshot to be a part of task while animating
app transition, and then remove it with DC#removeImeScreenshotIfNeeded
when the transition animation finished or no longer used gracefully.
 
Bug: 166736352
Bug: 153145997
Bug: 172815805
Bug: 174222049
Bug: 167604724

Test: manual as below steps:
   1) Launch an app with focusing an editor (e.g. Dialer)
   2) Swipe down status bar and tap Settings icon.
   3) Verify that when doing task transition animation, app activity
      with IME keeps visible.
Test: manual as below steps:
   1) With 2-button or 3-button gesture, launch an app with focusing an
      editor to show soft-keyboard.
   2) Pressing home key
   3) Verify the IME screenshot keeps visible and animates with closing
      transition smoothly.

Change-Id: I6bef36c779a28777408576f57e5d1c67d5d48e3f