The bulk of Rust packages in external/ are OS agnostic and do not care
about individual platform features. This change is intended to reduce
the churn of repeatedly adding new `apex_available` attributes to crates
by defaulting to availability. When landing a new crate which is either
platform revision specific or has an unstable file format, prefer to
set `apex_available: ["//apex_available:platform"]` until you have
thought things through.
Bug: 174862583
Test: Regenerate with no apex_available, see universal availability
Test: Regenerate with apex_available: [], see []
Test: Regenerate with apex_available: [foo], see [foo]
Change-Id: I958597789bfc966caec043d051996284dab7a927
a9fb65d155