Add -Wshadow and fix it

There are some "shadow" errors in the code, which can be a problem when
libvintf is used by other project with -Wshadow.

To avoid it:
- added -Werror to libvintf
- fixed the current violations

Bug: n/a
Test: m
Change-Id: Iff415cc31edf936ece65638c21ced9fcdfb9bbb8
4 files changed