Stop trying to detect when the soft keyboard is up

The code was hacky, made assumptions about the size of the IME UI, and
isn't actually working right now.  The platform doesn't want to tell us
when the keyboard is up; we need to accept that limitation.
Update display scale/position when canvas is resized
Drop unused return value of RemoteCanvas.pan()
Move position clamping to scrollToAbsolute()

Ensure the desktop is prevented from panning off the screen in every
code path that can update the view position.
Drop rotationCorrector

It shouldn't be needed anymore.
Make some RemoteCanvas members private
Center desktop image in the canvas