Rename message xml drawable so it's found on ldpi (2/2)

The ic_message_24dp.xml drawable is not found on ldpi devices.

The difference between ic_message_24dp and other xml drawables
which name a png src in a bitmap element (in order to also set
the autoMirrored attribute) is that in other instances
(e.g. ic_search_video_call.xml and ic_person_add_tinted_24dp.xml)
the png that is referenced has a different name from the xml
drawable (ic_videocam.png and ic_person_add_24dp for the previous
two examples)

Bug 27047302

Change-Id: I32843ca750aed57cc7c2586fa18cb02fe26192ef