Refactoring IME snapshot with adopting shell-transition

Extracts logic into the new ImeScreenshot class with using
TransitionController#{inTranstion, inRecentsTransition}
checks to attach/detach IME screenshot.

Bug: 201139553
Bug: 212570341
Test: atest DisplayContentTests#\
       testAttachAndShowImeScreenshotOnTarget
Test: manual as steps
   0) Settings -> Gestures -> enabling 3-buttons gesture mode
   1) Launch google search bar from the launcher to show IME
   2) Press home key and ensure IME screenshot is still visible
      when the google search activity is leaving w/w.o. enabling
      shell-transition.

Change-Id: I9228740cbf060a01fe985eb37c94c6abae6f79f8