Add legacy-test as replacement for core-junit

Statically includes core-junit-static into legacy-test target so
that legacy-test can be used in place of core-test. Once that
switch over has been done then android.test classes that are
being removed from the public API can be safely moved into here.

(cherry picked from commit bc90fcb983bb8d8066d50de90af5190a07c0a7f4)

Bug: 30188076
Test: make legacy-test and check contents of generated legacy-test artifacts
Change-Id: I7cdbeff5d95473ec2bfe6341d3957cf38d98e933