Fix issue #7457380: IME leaves a mark after user switching

The gnarly stuff where we keep track of the old input method
window as if it was still there was sitting around leaving things
in a stuck state.  Now we clear this out at key points in the
window manager (freezing screen, user change), and the input
method manager service is less aggressive about asking the window
manager to do it.

Also fixed a problem that was causing flickers during some
wallpaper transitions -- when we are animating two things on
top of the wallpaper and one of them disappears, we need to
make sure the wallpaper target points to whatever the current
target should be (if any), not left pointing to the old target
that has gone away.

Change-Id: I2fb9600f569a5bd5e3528aaf24cde9340af56cb0