Cleanup stale comments inside PolicyDefinition.java

There were 2 types of stale comments:
 * Comments describing the `null` behavior of @NonNull parameters
 * Comments talking about sPolicyDefinitions which no longer exists

Test: N/A, only changes to comments.
Flag: DOCS_ONLY
Bug: b/367311608

Change-Id: I2cf5c15624f68fc7d515be680af8b283a5078c65