with softinput_mode="pan", the window no longer pans back down when the keyboard is dismissed

reverting the following commit fixes the issue: https://github.com/kivy/python-for-android/commit/52e1861ca58657324a9f5452b1c91aaeca3963f3

when the keyboard is dismissed `heightDiff==0` and the control sequences are not sent.
