Get libgtest from libart-gtest.

We can't link libgtest into both the tests and libart-gtest because
gtest has global data that causes ODR issues. Instead, link libgtest
*whole* into libart-gtest to be sure that we actually have enough to
gtest. Headers from LOCAL_WHOLE_STATIC_LIBRARIES are re-exported
automatically, which solves the initial problem I was here for.

Test: mma test-art-host-gtest
Bug: None
Change-Id: Icb985e070d8afb3e614b4e94714c61bc95e4e7fc