Add legacy-android-test target containing android.test classes

In preparation for removing junit classes from the Android API
the legacy-test target will be removed from the
TARGET_DEFAULT_JAVA_LIBRARIES. This change adds a
legacy-android-test target, containing only the android.test
classes, for use by any targets that depend on those classes.

Bug: 30188076
Test: make checkbuild
Change-Id: Ifc0a9a23d4ac3c78428bb9dde38e547e6978ba7e