Fix issue with internal Active Source state tracking

Only set the internal Active Source device after the <Active Source>
message has been sent.

This prevents an issue where the internal state of a Source device
gets set with an incorrect logical address while address allocation is
still in progress.

This should in turn resolve issues with source devices not sending
Standby messages as they might believe they are not active source (due
to invalid LA).

Bug: 158441116
Test: atest HdmiCecLocalDevicePlaybackTest
Change-Id: Ibd78925a8bdc826341068c89e7f621f58dd41933