Do not assign InputMethod to non-input windows.

The InputMethod window was being assigned to the bottommost
window as a fall-through situation when no window could receive
input. This fix changes that so that if the bottommost window
cannot receive IME input then the InputMethod will be unassigned.

In most ANRs associated with bug 10689184 mInputMethodTarget
was assigned to the Wallpaper window and the InputMethod was
placed directly above it. When the ANR occurred the window that
was awaiting focused was then always placed immediately above the
InputMethod. This fix will keep that situation from happening.

Change-Id: Ic247e8132a907f2712a9f8a89e43c099142ec851