Let MCIMMS warn if per-display focus is not enabled

With a recent CL [1], per-display focus is now configurable and not
guaranteed to be enabled.  On the other hand, multi-client IME
framework always expects that per-display focus is enabled.

In order to not confuse developers, with this CL the system will post
a notification when such a mismatch is found.  This CL also clarifies
this point in the developer document.

There should be no behavior changes unless multi-client IME mode is
explicitly enabled.

 [1]: Ie030eed523599b217060887171710692d050e5d8
      51c5a1d0

Fix: 126909664
Test: Manually verified as follows.
  1. Build aosp_blueline-userdebug and flash it.
  2. make -j MultiClientInputMethod
  3. adb install -r \
     $OUT/system/priv-app/MultiClientInputMethod/MultiClientInputMethod.apk
  4. adb root
  5. adb shell setprop persist.debug.multi_client_ime \
     com.example.android.multiclientinputmethod/.MultiClientInputMethod
  6. adb reboot
  7. Make sure that a notification is shown and says that
     "config_perDisplayFocusEnabled" needs to be true in the system image.
Change-Id: I881ad57dc6478aabc0e8d7d031196cb2cc48a96d