KeyMint HALs: clarifications

 - Remove KeyPurpose comments that refer to public key operations.
 - Clarify/fix description of RSA_OAEP_MGF_DIGEST.
 - Describe HMAC key requirements.
 - Clarify RSA_PSS key length requirement
 - Clarify when shared secret should change (on restart)
 - Padding::NONE is not deprecated
 - Fix typos

Test: none, just comment changes
Change-Id: If58e8d8644aac926a990e50f7a873dca74cd4896