lmkd unsolicited notifications can cause lmkd to block if clients are not
consuming them. Fix that by sending notifications to only subscribed
clients. Introduce LMK_SUBSCRIBE command to allow lmkd clients to subscribe
to event notifications. The only asynchronous event currently supported is
LMK_ASYNC_EVENT_KILL.
Bug: 146597855
Test: fill up send buffer using lmkd_unit_test
Test: confirm lmkd does not block after the fix
Change-Id: I014159aa55b59081f4b9ed53ecd160a49c0682bb
Signed-off-by: Suren Baghdasaryan <surenb@google.com>