Upgrade android embedding to v2 (#60)

Co-authored-by: Jonas Franz <info@jonasfranz.software>
Reviewed-on: #60
Reviewed-by: konrad <konrad@kola-entertainments.de>
Co-authored-by: JonasFranz <email@jfdev.de>
Co-committed-by: JonasFranz <email@jfdev.de>

android/app/src/main/AndroidManifest.xml

@@ -13,7 +13,6 @@
          additional functionality it is fine to subclass or reimplement
          FlutterApplication and put your custom class here. -->
     <application
-        android:name="io.flutter.app.FlutterApplication"
         android:label="Vikunja"
         android:icon="@mipmap/ic_launcher">
         <activity
@@ -23,17 +22,13 @@
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density"
             android:hardwareAccelerated="true"
             android:windowSoftInputMode="adjustResize">
-            <!-- This keeps the window background of the activity showing
-                 until Flutter renders its first frame. It can be removed if
-                 there is no splash screen (such as the default splash screen
-                 defined in @style/LaunchTheme). -->
-            <meta-data
-                android:name="io.flutter.app.android.SplashScreenUntilFirstFrame"
-                android:value="true" />
             <intent-filter>
                 <action android:name="android.intent.action.MAIN"/>
                 <category android:name="android.intent.category.LAUNCHER"/>
             </intent-filter>
+            <meta-data
+                android:name="flutterEmbedding"
+                android:value="2" />
         </activity>
     </application>
 </manifest>

android/app/src/main/kotlin/io/vikunja/flutteringvikunja/MainActivity.kt

@@ -1,13 +1,5 @@
 package io.vikunja.flutteringvikunja
 
-import android.os.Bundle
+import io.flutter.embedding.android.FlutterActivity
 
-import io.flutter.app.FlutterActivity
-import io.flutter.plugins.GeneratedPluginRegistrant
-
-class MainActivity(): FlutterActivity() {
-  override fun onCreate(savedInstanceState: Bundle?) {
-    super.onCreate(savedInstanceState)
-    GeneratedPluginRegistrant.registerWith(this)
-  }
-}
+class MainActivity : FlutterActivity()