KeyMint VTS: local RSA encryption

Change RSA encryption (with public key) so it happens locally in the
test, rather than by invoking an ENCRYPT operation against KeyMint.

 - Specify MGF1 digest for OAEP mode as (now) required by AIDL spec.
 - Drop tests for too-long encryption inputs.
 - Adjust test comments to reflect decryption-only nature.
 - Change parameter checking tests to do so on DECRYPT rather than ENCRYPT.

Test: VtsAidlKeyMintTargetTest
Change-Id: I10c4beea28387eecfd0bc7c5dfd59a1b66fec21e