Support showing connected devices from system tethering

Also fixes IP address not showing up.

mobile/src/main/java/be/mygod/vpnhotspot/net/IpNeighbour.kt

@@ -0,0 +1,43 @@
+package be.mygod.vpnhotspot.net
+
+import android.util.Log
+import be.mygod.vpnhotspot.debugLog
+
+data class IpNeighbour(val ip: String, val dev: String, val lladdr: String) {
+    enum class State {
+        INCOMPLETE, VALID, VALID_DELAY, FAILED, DELETING
+    }
+
+    companion object {
+        private const val TAG = "IpNeighbour"
+
+        /**
+         * Parser based on:
+         *   https://android.googlesource.com/platform/external/iproute2/+/ad0a6a2/ip/ipneigh.c#194
+         *   https://people.cs.clemson.edu/~westall/853/notes/arpstate.pdf
+         * Assumptions: IPv4 only, RTM_GETNEIGH is never used and show_stats = 0
+         */
+        private val parser =
+                "^(Deleted )?((.+?) )?(dev (.+?) )?(lladdr (.+?))?( proxy)?( ([INCOMPLET,RAHBSDYF]+))?\$".toRegex()
+        fun parse(line: String): Pair<IpNeighbour, State>? {
+            val match = parser.matchEntire(line)
+            if (match == null) {
+                if (!line.isBlank()) Log.w(TAG, line)
+                return null
+            }
+            val neighbour = IpNeighbour(match.groupValues[3], match.groupValues[5], match.groupValues[7])
+            val state = if (match.groupValues[1].isNotEmpty()) State.DELETING else when (match.groupValues[10]) {
+                "", "INCOMPLETE" -> State.INCOMPLETE
+                "REACHABLE", "STALE", "PROBE", "PERMANENT" -> State.VALID
+                "DELAY" -> State.VALID_DELAY
+                "FAILED" -> State.FAILED
+                "NOARP" -> return null  // skip
+                else -> {
+                    Log.w(TAG, "Unknown state encountered: ${match.groupValues[10]}")
+                    return null
+                }
+            }
+            return Pair(neighbour, state)
+        }
+    }
+}