Add new file name constant for public DEX API file

We need to generate a new file from Doclava. Create a global variable
for the file name.

Bug: 64382372
Bug: 80461842
Test: make out/target/common/obj/PACKAGING/hiddenapi-blacklist.txt
Merged-In: Id850327c29bc626d3d721935edcb2b7afd2fd283
Change-Id: Icd0de1361879b896fd9d3881e405def98c012f0c