Properly package JNI libs in Robo tests

Previously, the jni_libs property was ignored when packaging
robolectric-tests.zip, meaning that tests sometimes couldn't find native
libraries. This change causes any native libs that tests use to be
written alongside the .jar/.apk for Robolectric tests.

99% of this code is just stolen from the Ravenwood tests, as they work
in this area. In short though, any native libs will be built for the
host arch and put in the correct place.

Tests are a little lacking (just testing the new functionality), but I'm
not 100% sure how to test the rest of the Robolectric file.

Bug: 340801983
Test: Manually verified, `go test` in java/
Change-Id: I4ef28b1cbc4675fc329dc8fcab06d055d6538e60