Handles capex in signing script

Today, the signing script simply ignores capex files, because it
looks for hardcoded '.apex' suffix. Add support to handle capex
as well.

Bug: 190574334
Test: Sign a target file locally
Change-Id: I3085ca7b0396a4fbf1b220f7de44d4eafb60c3d8
(cherry picked from commit 4d48d500)