Add flags needed to switch Jack to 1.8 for CTS targets

When linking against .jack files that contain 1.8 language
features Jack (currently) requires -D jack.java.source.version=1.8.

"core-tests" includes 1.8 language features.

Bug: 27113322
Bug: 26953739
(cherry-picked from commit 3269825f0be29ec8ea08e0b7930d801f3d0206b4)

Change-Id: I294b50edb898699b57684eaf25016553338cb6a2