HDMICEC: Avoid sending undesired <Active Source> messages

The bug occurs if a playback device wakes up from the standby state, trigerring CEC initialization,
when a message of type <Routing Change> or <Set Stream Path> is
requesting it to become the active source. Because the playback device
hasn't finished allocating LAs to local devices, the
ActiveSourceAction triggered by the previous message is deferred. Before
it has a LA allocated, the playback device ignores incoming messages of type <Routing Change> or <Set Stream Path>, not being aware of any active source changes.

This fix buffers the <Routing Change> and <Set Stream Path> messages.
After LAs allocation is done, the playback device can cancel any
queued ActiveSourceAction if there are any messages of these types or <Active Source> in
the buffer.

Bug: 235434112
Test: make && atest HdmiCecLocalDevicePlaybackTest
Change-Id: I318e090dcdb57ab023a1b401ec2503a21955007b