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
diff --git a/Android.mk b/Android.mk
index 514ed47..62d8b77 100644
--- a/Android.mk
+++ b/Android.mk
@@ -515,7 +515,7 @@
 LOCAL_ADDITIONAL_DEPENDENCIES := $(framework_res_R_stamp)
 
 LOCAL_NO_STANDARD_LIBRARIES := true
-LOCAL_JAVA_LIBRARIES := core-oj core-libart conscrypt okhttp core-junit bouncycastle ext
+LOCAL_JAVA_LIBRARIES := core-oj core-libart conscrypt okhttp legacy-test bouncycastle ext
 
 LOCAL_STATIC_JAVA_LIBRARIES :=                          \
     framework-protos                                    \