android: Default textColor of #000000 on Android.

Part of the RN v0.59 -> v0.60 changes to the template app [1]. This
purely cosmetic change may freely be positioned before the upgrade.

Corresponds to facebook/react-native@0851d5fac, released in RN
v0.60.0.

It looks like this was done for consistency among manufacturers, and
consistency with iOS; iOS uses #000000.

[1]: https://react-native-community.github.io/upgrade-helper/?from=0.59.10&to=0.60.6

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

@@ -5,6 +5,7 @@
         <!-- Customize your theme here. -->
         <item name="colorPrimary">@color/primaryColor</item>
         <item name="colorPrimaryDark">@color/primaryColorDark</item>
+        <item name="android:textColor">#000000</item>
     </style>
 
 </resources>