Implement signature check.

Currently, we just have debug keys, and always fail verification on
user builds. Production keys will be added later.

This CL also includes some helper scripts:
- Used to generate debug keys, for the record
- To sign data using the debug keys
- To verify base64 encoded data, used for debugging

Test: atest CtsSignedConfigHostTestCases
Note: The test also relies on some other changes going in too; it has
been verified with all relevant change in place, but will not pass at
HEAD quite yet.

Bug: 110509075
Change-Id: I8bd420c44a0a523cbefb21f90c49550c25beb0a6