hack: limit chat field length to 255

The common code truncates to 255 *bytes*, not chars, so this isn't even
right, but at least for ascii users it'll make it less likely that
what's typed doesn't all get sent. The right fix is to change the format
to not limit sent str len to 255 bytes. See commit 512a8e1af.

xwords4/android/app/src/main/res/layout/chat.xml

@@ -34,6 +34,7 @@
               android:layout_weight="1"
               android:scrollHorizontally="false"
               android:hint="@string/chat_hint"
+              android:maxLength="255"
               />
     <Button android:id="@+id/chat_send"
             android:layout_width="wrap_content"