Fix a bug where SpdySession.getActiveStreams() returns incorrect set

Related issue: #2742

Motivation:

When there are more than one stream with the same priority, the set
returned by SpdySession.getActiveStream() will not include all of them,
because it uses TreeSet and only compares the priority of streams. If
two different streams have the same priority, one of them will be
discarded by TreeSet.

Modification:

- Rename getActiveStreams() to activeStreams()
- Replace PriorityComparator with StreamComparator

Result:

Two different streams with the same priority are compared correctly.