Fixes an issue where Hex colors weren’t properly serialzed

- previously, the full ARGB was serialized, even though only RGB is
  supported

app/src/main/java/de/kuschku/quasseldroid_ng/util/irc/format/IrcFormatSerializer.kt

@@ -100,7 +100,7 @@ class IrcFormatSerializer internal constructor(private val context: Context) {
     fun writeHexColor(foreground: Int?) {
       out.append(CODE_HEXCOLOR)
       if (foreground != null) {
-        out.append(String.format(Locale.US, "%06x", foreground))
+        out.append(String.format(Locale.US, "%06x", foreground and 0x00FFFFFF))
       }
     }