Add new flag to generate public DEX API file

We need to tweak our dark greylist generation rule to include non-SDK
APIs "related" to the public API. Add a Doclava option to generate the
public API file as a list of DEX signatures.

Bug: 80461842
Bug: 64382372
Test: make out/target/common/obj/PACKAGING/hiddenapi-blacklist.txt
Change-Id: I5f92a54d718749f8ddef9472f3b6387a3debd385