[cec] power on TV and AVR if got active source

I don't expect that this PR will be accepted, but I don't find another way to get in touch with someone who knows how to do it right.

This patch fixes this issue:
my TV and my AVR are both powered off. XBMC is running. I start playing a movie with XBMC via an Android app. XBMC will power on the TV, but not the AVR so there is no sound and only live TV, because the AVR doesn't route the Picture from XBMC to the TV.

My change simply does on setActiveSource the same what is happening on start of XBMC.
This works just fine.

It would be nice if someone would be so kind to tell me what would be the right way to fix this issue.

Regards
Gerald