Fix a bug that 'LOCAL_MODULE_STEM'ed apks are not signed

LOCAL_MODULE_STEM is used to rename module or package names in the
final output system image. However, $(APKCERTS_FILE): target did not
support this LOCAL_MODULE_STEM scheme.

This commit fixes this bug by explicitly addressing
'LOCAL_MODULE_STEM'ed apk name correctly.

Change-Id: I4cf096266d97180a32ae054d6ff6cf2b7e5d042a