Conversation view touch event shenanigans

* Fix scrolling when the touch begins on an overlay view. Drags
  do not engage scrolling until the system touch-slop distance
  is exceeded. Logic copied from ScrollView.
* Fix pinch zoom when the first touch begins on the WebView by
  disabling motion event splitting.
* Fix pinch zoom when one or more touches begin on an overlay
  view by intercepting upon ACTION_POINTER_DOWN events.

Change-Id: I6f23eb75f9df32ac9c9b0c1be8a304e2c16c6fe0
5 files changed