scroll text field with touch

Add a hidden public method to text/method/Touch.java that
computes the maximum scroll amount for a text field.

Remove 'faketouch' code; it causes a crash and is
no longer required.

Pass the percentage of the current scroll from the UI
thread to webkit. One additional fix is to allow very
small movements which are currently disallowed because
they are smaller than 'smallerSlop' in WebTextView.java.

Companion fix is in external/webkit.

Fixes http://b/issue?id=2133049