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 a835180b)

Change-Id: I2be4db6788b14558f74534c24bb007f7fb8b6b67