Use a separate apk module definition to add core-tests resource.

Bug: 5264671
With this change the package.apk.unaligned is consistent with
package.apk.
Meanwhile it fixes a makefile bug:
Previously the rule updated both the target and the prerequisite.
Such behavors confuse make that we should avoid.

Change-Id: I1ce40a50450136355aa6489f82ad5ff29b82709c