rust: Refactor cfg and feature flag calculation

Move the cfg and feature flag calculation out of compilerFlags so that
it's a separate step.

The previous arrangement resulted in overridden compilerFlags which
must to set any additional cfgs/features before calling the base.
This is a bit confusing and undocumented behavior, so instead break
it out into a separate call that can itself be overriden.

Bug: N/A
Test: Soong tests pass
Change-Id: I28e4f707b3b3ca6eb621b7613c3737817f877bb8