Replace hiddenapi-{public,private}-list.txt with a CSV file

`hiddenapi` is being refactored to work with a single CSV file as
opposued to a multitude of text files (one per flag). This patch
changes the singleton rule for listing public/private APIs from
stubs to expect a CSV as an output.

Bug: 119068555
Test: compiles, hiddenapi-flags.csv unchanged
Change-Id: I622521d59979a6b01ecc8065a278412fedf7b9bc