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: I640b2ae6eeec17cab5cbc0bf7a4d54c4e4d35ccd