Remove isAnimating check in assignRelativeLayerForIme

Originally checking animating logic added in assignRelativeLayerForIme
from CL[1] is to fix IME flickering when playing app exit transition
while closing IME animation is not synchronized.

The original fix is not set RelZ to promote IME layer on top of the
IME target during animating in case seen IME animating while closing
the app.

Now with better IME transition, IME surface parent can be
reparented/attached to the activity surface, also with IME screenshot
mechanism we can guarantee that IME screenshot will still remain on
top of the activity while pressing home key to close the app.

Beside, adding isAnimating logic somehow raised the risk that may not
set relZ correctly if the app transition duration took longer like
the issue case is IME behind the dialog without setting relZ because
the longer default activity transition duration in T (400ms -> 450ms)

As the above reason, simply removing the isAnimating check to fix this
IME layering issue.

[1]: I0fb66b4cab8aa474c7a64d0cefd32310151d544e

Fix: 220960733
Test: atest ZOrderingTests
Test: atest FlickerTest#CloseImeWindowToHomeTest
Test: manual as test steps:
  0) Open Setup Wizard activity
  1) Defering the setup with touching on 4 corners of the screen
  2) Go to Settings -> Display -> turn on/off Dark theme
  3) Scroll down the notification then click Pixel Setup
  4) Select the wifi network, expect IME is shown on top of the wifi
     dialog

Change-Id: I93abda73c80e159dece6c6908414d39ae56b5a6e