Tweaks to libdexfile build definitions.

Remove vendor variant which I added in previous CL. Thanks to
David Sehr and Dan Willemsen for showing me how to avoid it.

Don't export headers. Our elf.h shadows system elf.h which breaks
most tools that depend on this library. Icky.

This is primarily to unblock development. Long term, we should
decouple the dex code more to avoid having to do this.

Bug: 66919073
Test: m test-art-host-gtest
Change-Id: Ie1d4a22754cec1d063e1644cd8ed1534e732bb1a