Specify media type for interface messages

Certain interface messages always refers to 'video', even when the
media is an audio file.

Create share_this_audio, rename download_full to download_full_video
and create download_full_audio.

Bug: T46267
Change-Id: I5826fce0d8b2d17cabaa3f0ce33543e3ffeb94fc
Merge "Specify media type for interface messages"