Make javac checkbuilds match jack

Building all java to dex is very slow.  Make javac checkbuilds act
like jack checkbuilds and only build to class files for unused
modules, and add a java-dex target that can be used to build
everything to dex.

Bug: 33105206
Test: m -j checkbuild
Change-Id: Idcf9c9e0223b9dd88aaa53beb0cb50a230034b0f
Merged-In: Idcf9c9e0223b9dd88aaa53beb0cb50a230034b0f
(cherry picked from commit 9eff830f)