Enabling New Insets API with Multi-Session IME.

New insets based IME requires windows manager to properly link the
assocaited IME window token against the target window token.

Test script:
Step 1: Patch this CL on rvc-dev repo
Step 2: Build and flash Hawk
Step 3: Build and install a sample multi-client IME
  make -j MultiClientInputMethod
  adb install -r $OUT/system/priv-app/MultiClientInputMethod/MultiClientInputMethod.apk
Step 4: Enable multi-client IME for the side-loaded sample multi-client IME
  adb root
  adb shell setprop persist.debug.multi_client_ime com.example.android.multiclientinputmethod/.MultiClientInputMethod
  adb reboot
Step 5: Open KitchenSync app, select carboard, enter text on Plain Text
tab (sample multi-session keyboard doesn't support numeric keyboard yet)

Bug: 123718661
Test: manual (see test script above)
Change-Id: I2a27c0da3a243d9cb3c98541531396cd9ff819a0