Specify MaterialIcons for react-native-vector-icons

I believe this reduces the app bundle size.

Closes #61

android/app/build.gradle

@@ -7,6 +7,10 @@ project.ext.react = [
     enableHermes: false,  // clean and rebuild if changing
 ]
 
+project.ext.vectoricons = [
+  iconFontNames: ['MaterialIcons.ttf']
+]
+
 apply from: "../../node_modules/react-native/react.gradle"
 apply from: "../../node_modules/react-native-vector-icons/fonts.gradle"