lmkd: Add optional kill reason description into kill reports

Allow kill report to be appended with the explanation of the reasons
killing has been done. This would help identify kill reasons while
troubleshooting lmkd kills.

Change-Id: Ie5dd7a44e51d04c43c2492be8c1bc964d1b03555
Signed-off-by: Suren Baghdasaryan <surenb@google.com>