java.security.spec: add public field in MGF1ParameterSpec

Pulling upstream changes from OpenJDK version jdk8u60 for
MGF1ParameterSpec.

Adding a public field for SHA224, defined in the same way
as existing ones for other SHA functions.

This completes the update of this package for jdk8u60.

Change-Id: I2416fb73050336a4a741c22bad3f2d740715647b
Test: make droid docs
Bug: 29631070