Create test-legacy/ for android.test.legacy target

The android.test.legacy (and legacy-android-test) target depends on
code from both test-base/ and test-runner/ and do not really belong
in either folder.

Having a separate folder will also provide a convenient place for
the artifacts needed to publish android.test.legacy to
maven.google.com.

Bug: 30188076
Test: make checkbuild

(cherry picked from commit 898e7de6)

Change-Id: I0538281980a55178dd72e5fae16d817cd31aa104