Tidy up application behaviour

Two changes here:

1. Clicking to stop the foreground service now exits the application.
2. The MainActivity launchMode is now singleTask.

Combined, these two gives a slightly more obvious behaviour when
starting the application and running it via intents.

app/src/main/AndroidManifest.xml

@@ -29,6 +29,7 @@
         tools:targetApi="31">
         <activity
             android:name=".MainActivity"
+            android:launchMode="singleTask"
             android:exported="true"
             android:theme="@style/SplashTheme.Soundscape">
 

app/src/main/java/org/scottishtecharmy/soundscape/MainActivity.kt

@@ -190,6 +190,8 @@ class MainActivity : AppCompatActivity() {
         } else {
             // service is already running, stop it
             soundscapeService?.stopForegroundService()
+            // And exit application
+            finishAndRemoveTask()
         }
     }