Add android.test.legacy target

Adds a library that builds against the public API and so can be safely
statically included into applications to avoid them having to depend
on the android.test.base and android.test.runner runtime libraries.

(cherry picked from commit a70f66cb)

Bug: 30188076
Test: make checkbuild
Change-Id: Ibd8cb61d00a65dbcf630672706323e42d82e6ba2
Merged-In: Iae7e3c64392e11035322092ed8e194740ba2d321