KeyMint VTS: test default MGF_DIGEST behaviour

KeyMint supports the specification of a separate MGF digest when
performing RSA-OAEP decryption, with a default value of SHA-1.

Test the expected behaviour here:
 - SHA-1 is used if nothing specified in key characteristics.
 - If something is specified in key characteristics, the operation
   parameter value has to be one of those values.

Bug: 203688354
Test: VtsAidlKeyMintTargetTest
Change-Id: Ic2dd3641be732a273724faa20cf4edf8a1752506