android: Set compile-/targetSdkVersion to 30

This will be mandatory for new apps in August and for existing apps
in November.  However, several classes like AsyncTask are now deprecated
so this needs some work to avoid warnings and problems in the future.

src/frontends/android/app/build.gradle

@@ -1,12 +1,12 @@
 apply plugin: 'com.android.application'
 
 android {
-    compileSdkVersion 29
+    compileSdkVersion 30
 
     defaultConfig {
         applicationId "org.strongswan.android"
         minSdkVersion 15
-        targetSdkVersion 29
+        targetSdkVersion 30
         versionCode 74
         versionName "2.3.2"
     }