Fix warnings reported by -Wshadow: Part1: aom_dsp directory

While we are at it:
- Rename some variables to more meaningful names
- Reuse some common consts from a header instead of redefining them.

Cherry-picked from aomedia/master: 09eea2193

Change-Id: I61030e773137ae107d3bd43556c0d5bb26f9dbf8