ObservableTunnel: do not cache key

Reported-by: Reza Island's <rezza.aji.ras@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

ui/src/main/java/com/wireguard/android/model/ObservableTunnel.kt

@@ -24,7 +24,8 @@ class ObservableTunnel internal constructor(
         config: Config?,
         state: Tunnel.State
 ) : BaseObservable(), Keyed<String>, Tunnel {
-    override val key = name
+    override val key
+        get() = name
 
     @Bindable
     override fun getName() = name