Disable all host OS variants except those explicitly included.

Necessary to avoid problems with implicitly enabled host OS'es, e.g.
linux_glibc getting enabled when we only supply a linux_bionic linker
in runtime-module-host-exports. That will then cause a non-functional
prebuilt to take precedence over source if the prebuilt is preferred.

We don't do this for device since we so far only support a single
device OS (android).

This introduces the notion that SDK member types can be host OS
dependent or not. That way java members with host prebuilts don't get
restricted to a specific host OS.

Test: m nothing
Test: build/soong/scripts/build-aml-prebuilts.sh runtime-module-host-exports
  Check that the generated Android.bp correctly disables the bionic
  linker prebuilt for linux_glibc.
Test: art/build/apex/runtests.sh
  on master-art with an updated runtime SDK snapshot
Test: art/tools/buildbot-build.sh {--host,--target}
  on master-art with an updated runtime SDK snapshot
Bug: 160349757
Change-Id: Idad7ef138cdbcbd209d390bf6c10ca8365d4619f