Don't allow IME windows to be IME targets

IME targets are required to be below the IME windows and that wouldn't
be possible if the IME window is its own target which leads to issues
elsewhere in the code where that is expected to be the case.

Change-Id: I34a723ff4ce2519cd80e0eea0eaea04712b6cf8f
Fixes: 33336368
Test: bit FrameworksServicesTests:com.android.server.wm.WindowStateTests