Limit Jack threads during tests

In default mode, run Jack with 4 threads to help reduce memory
footprint and thread contention. These are the same settings than
the Android build.

For the buildbot (where the Jack server is disabled), run Jack in
single-threaded mode to reduce the global memory consumption.

Bug: 31737153
Test: make -j32 test-art-host
Change-Id: I6a14efce92ea18dba008e6daa26cd6403e5eb503