android: un-prepare VPN service on revocation (#748)

vpnPrepared tracks whether Tailscale holds VPN permission. When permission is revoked, the VPNService must be reestablished. If vpnPrepared stays true, the Go backend reconnects, but startVPN() is not called in the ConnectView and the tunnel is not reestablished

Updates tailscale/tailscale#17190

Signed-off-by: kari-ts <kari@tailscale.com>

Author: kari-ts

android/src/main/java/com/tailscale/ipn/IPNService.kt

@@ -78,7 +78,7 @@ open class IPNService : VpnService(), libtailscale.IPNService {
           // This means that we were restarted after the service was killed
           // (potentially due to OOM).
           if (UninitializedApp.get().isAbleToStartVPN()) {
-            showForegroundNotification() 
+            showForegroundNotification()
             App.get()
             Libtailscale.requestVPN(this)
             START_STICKY
@@ -105,7 +105,10 @@ open class IPNService : VpnService(), libtailscale.IPNService {
   }
 
   override fun onRevoke() {
+    // VPN permission was granted to another app, so tell the Go backend and then set prepared to be
+    // false so that when user attempts to connect again, VpnService.prepare() is called
     app.setWantRunning(false)
+    setVpnPrepared(false)
     close()
     updateVpnStatus(false)
     super.onRevoke()