aapt: localizations: be more aggressive

* errors instead of warnings for:
  * A localized string for which no default-locale version exist.
  * A localized translation of an translateable="false" string.

Change-Id: Ib700fe323019f285f5a1931fe96500b2de03106b
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