VPN Hotspot 2.0: Client+ (#39)

Fix #13, #38. I don't have a lot of confidence that this would work very well for every device.

Also here's an SQL command that hopefully somebody could make into the app for me: `SELECT TrafficRecord.mac, SUM(TrafficRecord.sentPackets), SUM(TrafficRecord.sentBytes), SUM(TrafficRecord.receivedPackets), SUM(TrafficRecord.receivedBytes) FROM TrafficRecord LEFT JOIN TrafficRecord AS Next ON TrafficRecord.id = Next.previousId WHERE Next.id IS NULL GROUP BY TrafficRecord.mac;`

mobile/src/main/java/be/mygod/vpnhotspot/room/Converters.kt

@@ -0,0 +1,53 @@
+package be.mygod.vpnhotspot.room
+
+import android.os.Parcel
+import android.text.TextUtils
+import androidx.room.TypeConverter
+import java.net.InetAddress
+import java.nio.ByteBuffer
+import java.nio.ByteOrder
+
+class Converters {
+    @TypeConverter
+    fun persistCharSequence(cs: CharSequence): ByteArray {
+        val p = Parcel.obtain()
+        try {
+            TextUtils.writeToParcel(cs, p, 0)
+            return p.marshall()
+        } finally {
+            p.recycle()
+        }
+    }
+
+    @TypeConverter
+    fun unpersistCharSequence(data: ByteArray): CharSequence {
+        val p = Parcel.obtain()
+        try {
+            p.unmarshall(data, 0, data.size)
+            p.setDataPosition(0)
+            return TextUtils.CHAR_SEQUENCE_CREATOR.createFromParcel(p)
+        } finally {
+            p.recycle()
+        }
+    }
+
+    @TypeConverter
+    fun persistInetAddress(address: InetAddress): ByteArray = address.address
+
+    @TypeConverter
+    fun unpersistInetAddress(data: ByteArray): InetAddress = InetAddress.getByAddress(data)
+}
+
+fun String.macToLong(): Long = ByteBuffer.allocate(8).run {
+    order(ByteOrder.LITTLE_ENDIAN)
+    mark()
+    put(split(':').map { Integer.parseInt(it, 16).toByte() }.toByteArray())
+    reset()
+    long
+}
+
+fun Long.macToString(): String = ByteBuffer.allocate(8).run {
+    order(ByteOrder.LITTLE_ENDIAN)
+    putLong(this@macToString)
+    array().take(6).joinToString(":") { "%02x".format(it) }
+}