Added core-junit-static for use by frameworks/base/legacy-test

This is the same as core-junit but it is built for use in
LOCAL_STATIC_JAVA_LIBRARIES. It is intended to be used by
the legacy-test target and will be removed once the source files
for the legacy JUnit classes that are built into core-junit are
moved to frameworks/base/legacy-test.

Bug: 30188076
Test: make checkbuild cts droid
Change-Id: Ie0f441da14337669df4e6aa45c1e6b4bd59687dc