mp4: Replace the five tag value functions by a single one.

mp4: Replace the five tag value functions by a single one.

It's easier to let the caller pass the tag item string than to
have one caller for each of the five tags of interest. This commit
renames meta_find_by_name() to mp4_get_tag_value(), makes it public
and removes its five callers from mp4.c.

The only user is _aac_afh_get_taginfo() of aac_afh.c, which needs to
be adjusted accordingly. Kill the pointless underscore while at it.