Adding additional call to delete expired keys

Placing a call to delete all expired attestation keys directly in
the function responsible for retrieving them. This guarantees that any
key selected will be fresh. This also modifies
delete_expired_attestation_keys to create a time buffer so that a key
can't be milliseconds away from expiration when this call returns.

Bug: 220065678
Test:  atest keystore2_tests
Change-Id: I6f83eb65d02d8583d054c56ef6c572f3ee2e8e24
Merged-In: I6f83eb65d02d8583d054c56ef6c572f3ee2e8e24
1 file changed