Disallow setting LOCAL_MODULE_STEM for android packages

LOCAL_MODULE_STEM is not used in the packaging code,
except when setting PACKAGES.$(LOCAL_MODULE).STEM.
Having this disconnect between PACKAGES.$(LOCAL_MODULE).STEM
and the actual name of the apk causes an apk signing
step to fail.

Other module types like BUILD_HEADER_LIBRARY explicetly
disallow setting LOCAL_MODULE_STEM, so we can also just
do that for packages.

Fixes: 204027319
Test: Presubmits
Change-Id: Ib3da9a8dd6792a6c9d725cb24b835b32dbb13c40