Support new Doclava flag

CL I5f92a54d718749f8ddef9472f3b6387a3debd385 adds a new "-dexApi" flag
to Doclava, which generates the public API as a list of Dex signatures.
Add support for this flag to Soong and expose it in the droiddoc rule.

Bug: 80461842
Bug: 64382372
Test: make out/target/common/obj/PACKAGING/hiddenapi-blacklist.txt
Change-Id: Iadf09f17bb783209a6555ce5289c381bf025a01c