hiddenapi: Expect public/private API as CSV

Refactor of `hiddenapi` changed the output format of public/private API
lists to a single CSV file. Change API list generation accordingly.

In order to avoid special-casing this CSV file, it is treated the same
as the CSV files produced by `class2greylist`. The merging rules are
relaxed so that signatures in CSV files are not checked against
a pre-initialized set of all signatures (previously generated from the
public/private API files). This should not lead to build errors as the
CSV files are always auto-generated, and a missing/extra signature will
be caught by `hiddenapi`.

API lists in frameworks/base/config are processed later and checked
that they are a subset of the signatures in CSV.

Bug: 119068555
Test: compiles, hiddenapi-flags.csv unchanged
Change-Id: I33f2cbaa15f2d423a75e6ca64abe1c5b0c40c86f