APK Signature Scheme v3: add version number to proof-of-rotation.

The proof-of-rotation record contains a list of signing certificates
and corresponding flags.  New flags may be defined in future platform
versions, but APKs targeting P would have no knowledge of them.  Add
a version code to enable future platform versions to identify which
flags were deliberately set.  Ignore the version code for this
platform version, though, since all flags are known.

Bug: 64686581
Test: Builds, boots.
Change-Id: I765f50918f7f337100aff3ed15999b45369fc9d1