android: launch foreground service for starting vpn (#755)

Android 16 requires intents to start a foreground service for background starts. ACTION_START_VPN already calls startForeground()

Updates tailscale/tailscale#18847

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

Author: kari-ts

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

@@ -535,7 +535,7 @@ open class UninitializedApp : Application() {
     // FLAG_UPDATE_CURRENT ensures that if the intent is already pending, the existing intent will
     // be updated rather than creating multiple redundant instances.
     val pendingIntent =
-        PendingIntent.getService(
+        PendingIntent.getForegroundService(
             this,
             0,
             intent,