mdm: throw ErrNoSuchKey when a value not defined in Android syspolicy handler

Fixes tailscale/tailscale#11716

The implementation of the syspolicy handler on Android was incomplete. As explained in the comments (65f215115f/util/syspolicy/handler.go (L27)), a syspolicy handler should return a `syspolicy.ErrNoSuchKey` error when a value is not defined for a given key. The Android handler was instead returning an empty string.

When attempting to log in with a custom coordination server, since we were not returning syspolicy.ErrNoSuchKey, the caller into the syspolicy package (65f215115f/ipn/prefs.go (L665)) was always fetching an empty string from the MDM setting instead of using the default value `p.ControlURL`. Fallback logic would therefore always use the `DefaultControlURL` instead of the value defined in preferences.

Verified that upon specifying a custom coordination server in the app UI, a login screen for that coordination server appears and I can connect the Android device to that custom coordination server.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

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

@@ -453,17 +453,19 @@ class App : Application(), libtailscale.AppContext {
     return downloads
   }
 
-  @Throws(IOException::class, GeneralSecurityException::class)
+  @Throws(
+      IOException::class, GeneralSecurityException::class, MDMSettings.NoSuchKeyException::class)
   override fun getSyspolicyBooleanValue(key: String): Boolean {
     return getSyspolicyStringValue(key) == "true"
   }
 
-  @Throws(IOException::class, GeneralSecurityException::class)
+  @Throws(
+      IOException::class, GeneralSecurityException::class, MDMSettings.NoSuchKeyException::class)
   override fun getSyspolicyStringValue(key: String): String {
     return MDMSettings.allSettingsByKey[key]?.flow?.value?.toString()
         ?: run {
-          Log.d("MDM", "$key is not defined on Android. Returning empty.")
-          ""
+          Log.d("MDM", "$key is not defined on Android. Throwing NoSuchKeyException.")
+          throw MDMSettings.NoSuchKeyException()
         }
   }
 }

android/src/main/java/com/tailscale/ipn/mdm/MDMSettings.kt

@@ -11,6 +11,11 @@ import kotlin.reflect.full.isSubclassOf
 import kotlin.reflect.jvm.jvmErasure
 
 object MDMSettings {
+  // The String message used in this NoSuchKeyException must match the value of
+  // syspolicy.ErrNoSuchKey defined in Go, since the backend checks the value
+  // returned by the handler for equality using errors.Is().
+  class NoSuchKeyException : Exception("no such key")
+
   val forceEnabled = BooleanMDMSetting("ForceEnabled", "Force Enabled Connection Toggle")
 
   val exitNodeID = StringMDMSetting("ExitNodeID", "Forced Exit Node: Stable ID")