android: Move package namespace declaration from Manifest to build file

src/frontends/android/app/build.gradle

@@ -3,6 +3,8 @@ apply plugin: 'com.android.application'
 android {
     compileSdkVersion 30
 
+    namespace 'org.strongswan.android'
+
     defaultConfig {
         applicationId "org.strongswan.android"
         minSdkVersion 15

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

@@ -17,8 +17,7 @@
     for more details.
 -->
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    xmlns:tools="http://schemas.android.com/tools"
-    package="org.strongswan.android">
+    xmlns:tools="http://schemas.android.com/tools">
 
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />