Turn hidden API lists into a single CSV

Maintaining multiple text files has become too cumbersome as adding
each new category of API requires changes across many projects.
This patch changes generate_hiddenapi_lists.py to produce a single
CSV file in the format:

  <api_signature>,<flag1>,...,<flagN>

It can accept legacy API list files as input (for existing
frameworks/base/config/hiddenapi-*.txt files) as well as per-package
CSVs produced by class2greylist.

Test: m, check lists have not changed
Test: phone boots
Test: tools/hiddenapi/generate_hiddenapi_lists_test.py
Change-Id: Iebcef426ec93ea1d72b662bbff91d4e068fa0a70