Rename MyActivity to MainActivity

This will make react-native run-android work

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

@@ -10,7 +10,7 @@
         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

@@ -2,6 +2,6 @@ package com.example;
 
 import com.reactnativenavigation.controllers.SplashActivity;
 
-public class MyActivity extends SplashActivity {
+public class MainActivity extends SplashActivity {
 
 }