Move libadbd_auth, libadbd_fs to adbd_system_binaries.

The required block inside the definition of "adbd" does nothing: we get
the libraries it contains via direct dependency when building from
source, and not at all when using a prebuilt. Move them to a phony
rule that's explicitly listed in PRODUCT_PACKAGES.

Bug: http://b/157709367
Test: treehugger
Change-Id: I97c0889558482cfbe18ae91b39b6889d3fee877c