[fix] TouchableMixin async race condition
[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

necolas/react-native-web,necolas/react-native-web,necolas/react-native-web