Implement splash screen functionality

Adds a splash screen to improve the app's initial loading experience.

android/app/src/main/AndroidManifest.xml

@@ -1,3 +1,4 @@
+
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
 
@@ -15,7 +16,8 @@
             android:label="@string/title_activity_main"
             android:theme="@style/AppTheme.NoActionBarLaunch"
             android:launchMode="singleTask"
-            android:exported="true">
+            android:exported="true"
+            android:screenOrientation="portrait">
 
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />