Add a new (optional) trait that detects and converts keyblobs of a prior
format.  Invoke this trait during keyblob processing:

- Detect old keyblobs if keyblob parsing fails; if found, request key
  upgrade.
- On key upgrade, if keyblob parsing fails ask the trait impl to convert
  to current key format.

Test: TreeHugger
Bug: 197891150
Change-Id: I25480ee4d241b908c6e27ba9e4b8e34d86fc0b86