KeyStorage: improve logging for key generation

KeyStorage: improve logging for key generation

The error messages that are printed when probing for rollback resistance
support on a device that doesn't support rollback-resistant keys can
make it sound like something is going wrong.  Print a WARNING message
afterwards to try to make it clear what is going on.  Also adjust or add
DEBUG messages when starting to generate each key so that it's easier to
distinguish the log messages for different key generation operations.

Bug: 182815123
Test: boot on device that doesn't support rollback-resistant keys and
      check log.
Change-Id: I37a13eb5c1e839fb94581f3e7ec1cd8da0263d2b