Support compiling run-tests with jack

This CL adds support to compile run-test source files with jack. When
a test needs to rely on class files, we use jill to convert them to a
jack library.

We need to pass the full classpath to jack containing at least core
classes (like java.lang.Object). This means the Android tree must
have been compiled with jack first so we find all the necessary
classes.jack files.

Some tests still rely on dex files generated with the old toolchain.
We keep building them this way for the moment and will update them
later, when they get ready for Jack.

Also updates a few tests dealing with garbage collection to avoid a
situation where a reference can be retained by a local DEX register.

Bug: 19467889
Change-Id: I9eedd0705d1186d28a2acd37ea42a1762cd0ace2