aapt: allow toggling "errors instead of warnings"

* This patch makes the default behaviour same as AOSP,
  treating untranslatable strings in any but the default
  locale as warnings.
* Setting the environment variable AAPT_ERROR_ON_WARNING to
  anything will make those warnings, errors.

Signed-off-by: Chirayu Desai <cdesai@cyanogenmod.org>
Change-Id: I7da98d277495a7646ad64ba2f7e6a6bf14290f98