Rename MyActivity to MainActivity

This will make react-native run-android work

example/android/app/src/main/AndroidManifest.xml

         android:label="@string/app_name"
         android:theme="@style/AppTheme">
         <activity
-            android:name=".MyActivity"
+            android:name=".MainActivity"
             android:configChanges="keyboard|keyboardHidden|orientation|screenSize"
             android:label="@string/app_name">
             <intent-filter>

example/android/app/src/main/java/com/example/MyActivity.java → example/android/app/src/main/java/com/example/MainActivity.java

 
 import com.reactnativenavigation.controllers.SplashActivity;
 
-public class MyActivity extends SplashActivity {
+public class MainActivity extends SplashActivity {
 
 }