more complete 64bit mediaserver build targets

Change some mediaserver related build targets from "32" to "prefer32"
so that they match the mediaserver target itself. This positions us for
building 32bit in a 32+64 world and 64bit in a 64-only world.

Bug: 16890215
Test: build, check artifacts
Change-Id: If625b33f29f467ee111a8ec675b855322d26b5b2