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
Change-Id: Ied54c24694b3167fcf9075a3157e92ec53b8f636