android: fix Hostname type in MaskedPrefs, it should be String type (#482)

Author: yinkaisheng

android/src/main/java/com/tailscale/ipn/ui/model/Ipn.kt

@@ -162,7 +162,7 @@ class Ipn {
         ForceDaemonSet = true
       }
 
-    var Hostname: Boolean? = null
+    var Hostname: String? = null
       set(value) {
         field = value
         HostnameSet = true