G1DirtyCardQueueSet should use card counts rather than buffer counts

JDK-8227719 changed G1DirtyCardQueueSet to internally track the number of cards in the set, rather than the number of buffers, and provide that information to clients. But to reduce the fanout from that change, the old buffer count API was retained, with an estimated buffer count based on the card count and the buffer size.

We should update clients to use card counts and remove the buffer count API.