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: Ib9ceaadafc9c1d41ff137c48111ad3b060f7e248