[fix] TouchableMixin async race condition

When a Touchable component returns 0 for
`touchableGetHighlightDelayMS()` the mixin may occasionally try to
detect if the press event occurred on the hit area before the hit area
has been determined. Clearing out this value ensures that
`positionOnActivate` is recalculated before that takes place.

Close #586

src/components/Touchable/Touchable.js

@@ -399,6 +399,7 @@ const TouchableMixin = {
     if (delayMS !== 0) {
       this.touchableDelayTimeout = setTimeout(this._handleDelay.bind(this, e), delayMS);
     } else {
+      this.state.touchable.positionOnActivate = null;
       this._handleDelay(e);
     }