Improve IME flicker when switching to work-profile task

CL[1] using DC#mayImeShowOnLaunchingActivity to check if the opening
task has to defer tasksnapshot removal if the task is going to show IME.

However, the checks in mayImeShowOnLaunchingActivity may not predictable
when switching the work-profile task to home then back to this task,
since with per-profile IME, the IME may be unbound and the IME window will
be detroyed and no surface, so defer tasksnapshot didn't work and will see
the IME flickering as the above use case.

Simply remove unpredicable IME window state chacking and just using
AR#mLastImeShown state to ensure the defer removal works.

[1]: I81b048a655124923a5f19e7f236511502bbf4c91

Bug: 210797942
Test: manual as steps
  0. Login into work-profile account.
  1. Launch google chat of the profile user with focusing the editor
     to show IME
  2. Swipe out to the home screen.
  3. Launch google chat again, expect no flicker in the most cases (e.g.
     work-profile IME didn't be killed unexpected or longer IME cold
     start time > defer snapshot delayed time)

Change-Id: I45cff4a03522f59ab8a72adbbfd700e18577e2e6