Reworking keyboardShouldPersistTaps to have a middle ground

Summary:
Right now, the ScrollView's keyboard hiding behavior is either all or nothing: Hide the keyboard on any tap, or do nothing ever. This PR introduces a third mode to keyboardShouldPersistTaps which is much closer to what I consider should be the default.

In the new behavior, the tap responding is done in the bubbling phase (instead of the capture phase like =true). As a result, a child can handle the tap. If no child does, then the ScrollView will receive the tap and will hide the keyboard. As a result, changing TextInput focus works as a user expects, with a single tap and without keyboard hiding. But taping on Text or on the empty part of the ScrollView hides the keyboard and removes the focus.

You can view the behavior in a monkey patched ScrollView demo on rnplay:
https://rnplay.org/apps/E90UYw
https://rnplay.org/apps/UGzhKA

In order to have a uniform props set, i added 3 values to the keyboardShouldPersistTaps:
'never' and 'always' are the same as false and true.
'handled' is the new behavior.

I don't
Closes https://github.com/facebook/react-native/pull/10628

Differential Revision: D4294945

Pulled By: ericvicenti

fbshipit-source-id: 1a753014156cac1a23fabfa8e1faa9a768868ef2

Examples/UIExplorer/js/UIExplorerExampleList.js

@@ -82,7 +82,7 @@ class UIExplorerExampleList extends React.Component {
           renderRow={this._renderExampleRow.bind(this)}
           renderSectionHeader={this._renderSectionHeader}
           enableEmptySections={true}
-          keyboardShouldPersistTaps={true}
+          keyboardShouldPersistTaps="handled"
           automaticallyAdjustContentInsets={false}
           keyboardDismissMode="on-drag"
         />