lmkd: Add support for multiple lmkd client connections

lmkd testing requires communication with lmkd daemon to register new
native processes. New implementation allows more than one communication
channel to lmkd. Current max number of communication channels is set to
two - one for ActivityManager and another one for a test process.

Bug: 63631020

Change-Id: I736115938a3c5ad9253bce29a17cd5349af190eb
Signed-off-by: Suren Baghdasaryan <surenb@google.com>