Renaming "Observer" to "Listener" for consistency in HTTP/2

Motivation:

We currently have a mix of "Observer" and "Listener" for interface
names. We should make them all "Listener" to be consistent.

Modifications:

Renamed Http2DataObserver->Http2DataListener and
Http2FrameObserver->Http2FrameListener.

Result:

Listener interface names are consistent.