Extend attestation key expiration buffer to 12 hours

This buffer time is how far into the future keystore looks to mark
a key as "old" and needing deletion.

The old buffer time was only 20 seconds, which may not even be enough
for very slow network (where round trips and retries could take
minutes).

Test: keystore2_test
Change-Id: I27869458ec66e546b42fe344bbdffd32881c84e7
1 file changed