CEC: Handle <Active Source> from AVR properly

To avoid timing issues, handling of <Active Source> was delayed until
corresponding tv input becomes ready. However, AVR is not considered as
a tv input, so delayed <Active Source> will not be processed. This
change makes <Active Source> from AVR handled immediately rather than
buffered.

Bug: 25630805
Change-Id: Iae8e86a84b5a9e5a2e62e457aaf93da67f145fac