Add hiddenapi_flags module type

The build generates a file out/soong/hiddenapi/hiddenapi-flags.csv
which is used by the hiddenapi-blacklist tests in cts/tests/signature.
The generation is done at the ninja level and so is not accessible from
the soong layer that runs on top. This change adds the hiddenapi-flags
module type which makes the file accessible from other soong modules.

Bug: 122332514
Test: atest -p cts/tests/signature
Change-Id: If38c8a8ffca110f2ae01f97f19a2740ca3fde1b7
Exempt-From-Owner-Approval: Colin has already reviewed and only had minor
    nits and this is blocking other changes.