On-device signing: Delete the HMAC key when failing to use it.

This may allow us to recover in certain bad situations. Also, add some
more clear error logs when failing to create/delete a key, to make it
easier to debug failures.

Bug: 190711210
Test: TEST_MAPPING
Change-Id: Ib9a9ce0c0d0e99ce44d124af85775780f448a854
3 files changed