ART: Refactor run-test 912 (1/3)

Refactor the test for CTS. Break out the ART-specific part. Move
the rest into the regular layout. Move class events to their own
thread, and report them on the Java side. Use InMemoryDexClassLoader
for classloaders to avoid explicit files.

Bug: 32072923
Test: none
Change-Id: I80e27cc9c21646bbecd1de1a246f583a0d388e05