Implement onKeyPress Android

Summary:
This implements onKeyPress for Android on TextInputs and addresses facebook/react-native#1882.
**N.B. that this PR has not yet addressed hardware keyboard inputs**, but doing will be fairly trivial. The main challenge was doing this for soft keyboard inputs.

I've tried to match the style as much as I could. Will happily make any suggested edits be they architectural or stylistic design (edit: and of course implementation), but hopefully this is a good first pass :).
I think important to test this on the most popular keyboard types; maybe different languages too.
I have not yet added tests to test implementation, but will be happy to do that also.

- Build & run RNTester project for Android and open TextInput.
- Enter keys into 'event handling' TextInput.
- Verify that keys you enter appear in onKeyPress below the text input
- Test with autocorrect off, on same input and validate that results are the same.

Below is a gif of PR in action.
![onkeypressandroid](https://user-images.githubusercontent.com/1807207/27512892-3f95c098-5949-11e7-9364-3ce9437f7bb9.gif)
Closes facebook/react-native#14720

Differential Revision: D6661592

Pulled By: hramos

fbshipit-source-id: 5d53772dc2d127b002ea5fb84fa992934eb65a42

Author: joshjhargreaves

Diff for: js/TextInputExample.android.js

@@ -27,6 +27,7 @@ class TextEventsExample extends React.Component<{}, $FlowFixMeState> {
     curText: '<No Event>',
     prevText: '<No Event>',
     prev2Text: '<No Event>',
+    prev3Text: '<No Event>',
   };
 
   updateText = (text) => {
@@ -35,6 +36,7 @@ class TextEventsExample extends React.Component<{}, $FlowFixMeState> {
         curText: text,
         prevText: state.curText,
         prev2Text: state.prevText,
+        prev3Text: state.prev2Text,
       };
     });
   };
@@ -46,6 +48,7 @@ class TextEventsExample extends React.Component<{}, $FlowFixMeState> {
           autoCapitalize="none"
           placeholder="Enter text to see events"
           autoCorrect={false}
+          multiline
           onFocus={() => this.updateText('onFocus')}
           onBlur={() => this.updateText('onBlur')}
           onChange={(event) => this.updateText(
@@ -60,12 +63,16 @@ class TextEventsExample extends React.Component<{}, $FlowFixMeState> {
           onSubmitEditing={(event) => this.updateText(
             'onSubmitEditing text: ' + event.nativeEvent.text
           )}
+          onKeyPress={(event) => this.updateText(
+            'onKeyPress key: ' + event.nativeEvent.key
+          )}
           style={styles.singleLine}
         />
         <Text style={styles.eventLabel}>
           {this.state.curText}{'\n'}
           (prev: {this.state.prevText}){'\n'}
-          (prev2: {this.state.prev2Text})
+          (prev2: {this.state.prev2Text}){'\n'}
+          (prev3: {this.state.prev3Text})
         </Text>
       </View>
     );