Alter spec text for RSA-PSS to match reality

The Key{Mint,Master} spec previously said that RSA-PSS mode should use
SHA-1 for the MGF1 digest, separately from whatever Tag::DIGEST gets
specified as the main digest.

However, both the reference implementation and the VTS/CTS tests
use BoringSSL's defaults, which is to re-use the main digest as the MGF1
digest if none is separately specified.

Given that this behaviour is embedded in many implementations over
several years (and given that there isn't a security implication),
change the spec to match this behaviour.  Also update the VTS test
code to make this clear/obvious.

Test: VtsAidlKeyMintTargetTest, VtsHalKeymasterV4_0TargetTest
Bug: 210424594
Merged-In: I4303f28d094ef4d4b9dc931d6728b1fa040de20d
Change-Id: I4303f28d094ef4d4b9dc931d6728b1fa040de20d