remove common_flags += -Werror to fix compile

Change-Id: I7aa8a8d37658a56460a08de51600e38daa1ecb47
libhwc: Lower optimization level
Fix explicit instantiations

Summary:
This fixes explicit instantiations of android::Singleton.

Test Plan: verify

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D619
caf: hwcomposer: build with gcc

Summary: Workaround tricky inline functions

Test Plan: verified

Reviewers: jriordan001, randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D657