Remove support of disabling Jack.

We removed code and variables related to running dx on classes.jar in
this change. Also removed target emma rules (but kept the emma rules for
host java libraries), for it's now done by Jack.
We still support to build classes.jar (and javalib.jar for static Java
libraries) using javac, because tools like javadoc need class files as
input.
Removed the obsolete install-dex-debug.

Bug: 27400061
Change-Id: If0bcdfe62cb181a98754fb0dbe1c12c92e38d3e8