Camera2: C++11 fixes

Summary:
   - Fixed in-class constexpr handling
   - Fixed invalid conversion from 'int' to 'ImageType {aka unsigned
     char*}'
   - Fixed comparison between char pointer and int

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D100