Bug 5352720: IME_FLAG_NO_EXTRACT_UI is not enforced

CL https://android-git.corp.google.com/g/#/c/136474 refactored
the fullscreen test, but ithe IME_FLAG_NO_EXTRACT_UI test was
lost in the process.

Note that there is still a problem (orthogonal to that change
with key_preview_backing, which appears opaque and black sometimes.

I'll re-open 5315001.

Change-Id: If3a73179d21eaca10bdc948db7bac4b4f7a88d34