Default textColor of #000000 on Android (#24540)

Summary:
By default, the text color is `#000000` on iOS and different on Android, e.g. `#808080`, depending on the manufactorer.

This PR changes it so that newly created projects all have the text color `#000000` by default on both iOS and Android.

The argument for this is to make the app by default be more consistent between platforms.

Expo also does this: https://github.com/expo/expo/blob/master/android/expoview/src/main/res/values/styles.xml#L31

 ---

For context and for your consideration, I have started a discussion here with the topic of whether React Native should try to use OS defaults or be consistent between platforms:

https://github.com/react-native-community/discussions-and-proposals/issues/121

[Android] [Changed] - New projects have a `#000000` by default.
Pull Request resolved: https://github.com/facebook/react-native/pull/24540

Differential Revision: D15044898

Pulled By: cpojer

fbshipit-source-id: 3197266504e1061ac7027bec3100e39e39a4406a

RNTester/android/app/src/main/res/values/styles.xml

@@ -3,6 +3,7 @@
     <!-- Base application theme. -->
     <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
         <!-- Customize your theme here. -->
+        <item name="android:textColor">#000000</item>
     </style>
 
 </resources>

template/android/app/src/main/res/values/styles.xml

@@ -3,6 +3,7 @@
     <!-- Base application theme. -->
     <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
         <!-- Customize your theme here. -->
+        <item name="android:textColor">#000000</item>
     </style>
 
 </resources>