Check for MGF1 digests in key characteristics.

A bug in the Trusty HAL service caused it to replace MGF1 digest tags
with Tag::INVALID.  This tests that MGF1 tags are returned properly in
the MGF1 success test, and verifies that Tag::INVALID is never
returned by any test.

Bug: 278157584
Test: adb shell /data/nativetest/VtsAidlKeyMintTargetTest/VtsAidlKeyMintTargetTest
Change-Id: I5d391310795c99f37acf3c48310c127a7a31fac3