Allow APEXes permission XMLs to declare features.

This allows APEXes to include permission XMLs like the NFC XMLs in
frameworks/native/data/etc which declare NFC features.

(This also enables `unavailable-feature` declarations.)

Bug: 197680726
Test: Include an NFC permission XML in an APEX.
      Observe feature is set correctly in logs and by
      enabling NFC in Settings app.
Change-Id: Ied425b8897ab80df250d42cfbf0525634ed948aa