Keystore 2.0: Deprecate Credential prefixes.

Keystore 2.0 will no longer support free form blobs. Certificates and
certificate chains will have types fields associated with an alias.
Other free form blobs will need to be migrated to a different key value
store.

Bug: 171305684
Test: None
Change-Id: I93270f0086329229dc36c2b14c88f229351e6560