Refine systemfeature codegen behavior

Tweak the semantics of codegen tool feature version args, such that a
`--feature=$FEATURE:$VERSION` arg has the following $VERSION semantics:
  * ""            == undefined (runtime API)
  * valid int     == enabled   (compile-time API)
  * "UNAVAILABLE" == disabled  (compile-time API)

This will simplify how feature args get collected from build flags, and
also aligns with terminology used elsewhere for explicitly disabling
features via the "<unavailable-feature />" tag.

Bug: 203143243
Test: atest systemfeatures-gen-tests systemfeatures-gen-golden-tests
Flag: EXEMPT refactor
Change-Id: I502d455effbe4b81794a5f529406b8b68ac2b372