core: Move core.oat bootjar definitions into envsetup.mk

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

Also refactor PRODUCT_BOOT_JARS in core-tiny.mk and core-minimal.mk to reuse TARGET_CORE_JARS.
The above variables can be used from scripts with get_build_var from envsetup.sh

  $> get_build_var TARGET_CORE_JARS
  <prints list of core jars>

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

Bug: 36902714
Test: make -j32 build-art-host build-art-target
Test: lunch aosp_angler-userdebug && make -j32
Test: make -j32 PRODUCT-sdk_google_aw_x86-sdk
Change-Id: If12569719343bfbba20c1cda9a5302ab541dbfc6