Add android.test.legacy.jar to SDK build

This is needed in order to create a prebuilts version of this JAR that
can be used for unbundled builds.

Bug: 30188076
Test: wait for pi-release build and check

(cherry picked from commit af06ed42)

Change-Id: I0ea83bc88b6107f4154a9ca02f13b7c8cb367fea