Bug 955887 -  [Buri] "Add Collection" wallpaper label missing

the root cause is we didn't use foo.en-US.properties for en-US instead
of en-US/apps/foo/foo.properties if GAIA_CONCAT_LOCALES=1, so we need to
use the original file in gaia directory not in locale basedir.
Merge pull request #15393 from yurenju/bug955887

Bug 955887 -  [Buri] "Add Collection" wallpaper label missing r=@timdream