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.

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

Change-Id: I75488303067c4ac34c46279bfce9a51273adcc34