Decouple system feature API codegen from feature definitions

The set of build-time, readonly features will vary for each device. To
ensure we at least have a minimal static API surface to target, allow
specifying a minimal feature API set alongside any explicitly defined
feature values.

This will let us start migrating a subset of internal hasSystemFeature
API queries to use the new API, independent of integration with the
build system and per-device build flag settings.

Initially, this will just be a passthrough API surface with no build or
runtime behavioral differences. As we integrate per-device build flags,
the (sub)set of explicitly defined features will change how each API
behaves.

Bug: 203143243
Test: atest systemfeatures-gen-tests --host
Change-Id: I3c84375bdc1a2e31cc72fa09f3b980c2daa85bb2