Handles incoming commands <Active Source>, <Inactive Source>, <Request
Active Source> that concern the active source update/report.
Defined mPrevPortId which is used to allow switching back to the input
set previously by <Active Source> or deviceSelect() when receiving
<Inactive Source>.
Also fixed a bug sending <Active Source> with wrong parameters.
Change-Id: I47fd804755c5c6beca38ddb21bb388a455d74f63
8333571bd5