biometric: Add nullable to api for crypto

The javadocs mention that the functions getMac(), getKeyAgreement,
getCipher() can return nullable but this isn't show in the API.
Updating these to reflect that

Bug: 304697130
Test: atest FrameworksCoreTests
Change-Id: Icda5a087775174517abca7b4d1078a6ea0719170
3 files changed