core: Move core.oat bootjar definitions into core/config

TARGET_CORE_JARS and HOST_CORE_JARS define the minimal
bootclasspath jars necessary to run with dalvikvm/art.

Also refactor PRODUCT_BOOT_JARS to reuse TARGET_CORE_JARS.

(These changes are required to figure out what --bootclasspath_entry
to pass to desugar).

Bug: 36902714
Test: make -j32 && make -j32 build-art-host build-art-target
Change-Id: I620e28be41047a1d501d7f144fe415ee4a0af36f