Use -Werror in system/extras

* Remove unused variables.
* Fix redefined macro warnings.
* Fix sign-compare warnings.
* Fix 'return false' in main().
* Keep existing warnings to fix later.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: Ib92ef5297693595fb84ed4f8e61665bda4cee312
57 files changed