RN: Unify Typeface Logic (Android)

Summary:
Refactors how `Typeface` style and weight are applied in React Native on Android.

- Unifies all style and weight normalization logic into a new `TypefaceStyle` class.
  - Fixes font weight support for the Fabric renderer.
  - De-duplicates code with `TextAttributeProps`.
  - Simplified normalization logic.
- Fixes a rare crash due to `Typeface.sDefaultTypeface` (Android SDK) being `null`.
- Adds a new example to test font weights in `TextInput`.
- Adds missing `Nullsafe` and `Nullable` annotations.
- Clean up a bunch of obsolete inline comments.

Changelog:
[Android][Fixed] - Fixed a rare crash due to `Typeface.sDefaultTypeface` (Android SDK) being `null`.
[Android][Fixed] - Fixed font weight support for the Fabric renderer.
[Android][Added] - Added a new example to test font weights in `TextInput`.

Reviewed By: JoshuaGross

Differential Revision: D29631134

fbshipit-source-id: 3f227d84253104fa828a5561b77ba7a9cbc030c4

packages/rn-tester/js/examples/TextInput/TextInputExample.android.js

@@ -200,6 +200,38 @@ exports.examples = ([
       );
     },
   },
+  {
+    title: 'Font Weight',
+    render: function(): React.Node {
+      return (
+        <View>
+          <TextInput
+            defaultValue="Font Weight (default)"
+            style={[styles.singleLine]}
+          />
+          {[
+            'normal',
+            'bold',
+            '900',
+            '800',
+            '700',
+            '600',
+            '500',
+            '400',
+            '300',
+            '200',
+            '100',
+          ].map(fontWeight => (
+            <TextInput
+              defaultValue={`Font Weight (${fontWeight})`}
+              key={fontWeight}
+              style={[styles.singleLine, {fontWeight}]}
+            />
+          ))}
+        </View>
+      );
+    },
+  },
   {
     title: 'Text input, themes and heights',
     render: function(): React.Node {