Introduce a singleton module to collect apex certs

`all_apex_certs` will provide two kinds of output files:
1. x509 certificate in pem format
2. x509 certificate in der format

filenames of the certs are not part of `all_apex_certs` api. In fact,
the der certs will be named with int indexes.

This singleton module will be used by `CtsSecurityTestCases`
(specifically `PackageSignatureTest`) to enforce that the apexes are
signed with release keys.

To implement this, `ctx.ReverseDepenendcy` will be used in deps mutator
of apex. An alternative would have been to use `ctx.VisitAllDeps` in
`GenerateAndroidBuildActions` of the singleton, but this would make it
unusable in the cts test (circular dep).

Make has a similar implementation that collects the certs of apexes and
dists that file. This has been kept separate for now to prevent b/304914238

Bug: 329299639
Test: m nothing --no-skip-soong-tests
Change-Id: I742d8ae1ccc344a78ae04263382750508b2aedec