[Android] MODULE_TAGS

* LOCAL_MODULE_TAGS could be [user/debug/eng/tests/optional/samples/shell_ash/shell_mksh]
==> if wrong tag, the error msg is "unusual tags xxx on yyy at ...." in base_rules.mk

* TARGET_BUILD_VARIANT could be [user/userdebug/eng/tests]
==> when user , tags_to_install = user
==> when userdebug, tags_to_install = user debug
==> when eng (or sdk build), tags_to_install = user debug eng
==> when tests, tags_to_install = user debug eng tests
==> those "optional" modules are decided on declaring PRODUCT_PACKAGES
refer to main.mk