[CredMan] Prefetch credential get api

Based on partner feedback, add a new prefetch get-credential api that
supports first retrieving credential candidates and delaying the UI
flows to finalize a credential to later using a separate api,
GetPendingCredentialResponse.show().

Bug: 273308895
Test: local deployment (will add unit test with the unhiding change)
Change-Id: I212a6066c71b8468c6d7bfc2df2d9dab737368e0