tools: Read ANDROID_COMPILE_WITH_JACK from build

This centralizes switching the default usage of jack vs javac into the
build.

Run-tests changed to unconditionally use javac/dx by default
(but this doesn't affect automation since we always go through
testrunner).

Bug: 36902714
Change-Id: I4b733918c780878a300c0413668c53ad756038c4