Ensure that only one dex jar is chosen for each boot image library

This change does not break modules like "core-oj" that are in multiple
apexes, i.e. com.android.art, com.android.art.debug and
com.android.art.testing because they all shared the same variant. So,
the getBootImageJar() is only called once for that variant.

Test: m droid
Bug: 171061220
Change-Id: Ibf26da147af2b49ab9e4588030e8cd4002d04a7a