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
Change-Id: If40239d4af59a3d3f71b22e8ef2a07e553c8d5c2