When he’s not piggybacking on already established franchises (“Journey 2,” “Fast Five,” “G.I. Joe 2”), The Rock — sorry: Dwayne Johnson — periodically headlines shockingly dour programmers that are more interesting than they should be without being all that interesting. 2010’s not very aptly titled “Faster” was sold as a revenge spectacular, only to reveal itself as a brooding, if humorless, look at guilt and rot. The new “Snitch” is even slower, draggier, humor-impaired.