Capture Android back key

Works the same as back button in Header.

Change-Id: I89a4fa753281e060fcd9944059dcd9a1bab5081d
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Make it a bit more fault tolerant

This should allow ConferenceHeader to load even with missing
images server side.

Change-Id: Id808f3c30b95530ecef30b978a2580285fb67783
Reviewed-by: Caroline Chao <caroline.chao@digia.com>