Fix for starting drag on Views which handle down event

If you have a Button within a row in a ListView, you cannot start the drag from within the Button. This is because ACTION_DOWN is handled by the Button and not propagated to the PullToRefreshAttacher. My suggested fix allows the drag to be started from the ACTION_MOVE event too, if the initial position has not yet been set.

This is an old bug that was fixed, but it seems to have been undone after recent refactoring.

Fixes:
https://github.com/chrisbanes/ActionBar-PullToRefresh/issues/131
Original issue:
https://github.com/chrisbanes/ActionBar-PullToRefresh/issues/18
Original fix:
https://github.com/chrisbanes/ActionBar-PullToRefresh/commit/16843df464e92c28e4622d9eeb756ce2ab7eedb1