fix MissingForegroundServiceTypeException

The sample app is not yet well prepared for API34 and lacks the foreground service type declaration. When starting the app on API34 it crashes with:

FATAL EXCEPTION: main
Process: org.torproject.artitoyvpn, PID: 7028
java.lang.RuntimeException: Unable to start service org.torproject.artitoyvpn.vpn.ArtiVpnService@4f9cc3 with Intent { act=ArtiVpnService.start cmp=org.torproject.artitoyvpn/.vpn.ArtiVpnService }: android.app.MissingForegroundServiceTypeException: Starting FGS without a type  callerApp=ProcessRecord{4eeb107 7028:org.torproject.artitoyvpn/u0a194} targetSDK=34
	at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:4839)
	at android.app.ActivityThread.-$$Nest$mhandleServiceArgs(Unknown Source:0)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2289)
	at android.os.Handler.dispatchMessage(Handler.java:106)
	at android.os.Looper.loopOnce(Looper.java:205)
	at android.os.Looper.loop(Looper.java:294)
	at android.app.ActivityThread.main(ActivityThread.java:8177)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:971)
Caused by: android.app.MissingForegroundServiceTypeException: Starting FGS without a type  callerApp=ProcessRecord{4eeb107 7028:org.torproject.artitoyvpn/u0a194} targetSDK=34
	at android.app.MissingForegroundServiceTypeException$1.createFromParcel(MissingForegroundServiceTypeException.java:53)
	at android.app.MissingForegroundServiceTypeException$1.createFromParcel(MissingForegroundServiceTypeException.java:49)
	at android.os.Parcel.readParcelableInternal(Parcel.java:4870)
	at android.os.Parcel.readParcelable(Parcel.java:4852)
	at android.os.Parcel.createExceptionOrNull(Parcel.java:3052)
	at android.os.Parcel.createException(Parcel.java:3041)
	at android.os.Parcel.readException(Parcel.java:3024)
	at android.os.Parcel.readException(Parcel.java:2966)
	at android.app.IActivityManager$Stub$Proxy.setServiceForeground(IActivityManager.java:6761)
	at android.app.Service.startForeground(Service.java:775)
	at org.torproject.artitoyvpn.vpn.ArtiVpnService.onStartCommand(ArtiVpnService.java:97)
	at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:4821)
	at android.app.ActivityThread.-$$Nest$mhandleServiceArgs(Unknown Source:0) 
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2289) 
	at android.os.Handler.dispatchMessage(Handler.java:106) 
	at android.os.Looper.loopOnce(Looper.java:205) 
	at android.os.Looper.loop(Looper.java:294) 
	at android.app.ActivityThread.main(ActivityThread.java:8177) 
	at java.lang.reflect.Method.invoke(Native Method) 
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552) 
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:971)