Add start time to LmkKillOccurred

This is to measure an application's behavior with respect to being LMKed
(the longer an app lives before being LMKed, the better).

Bug: 119854389
Test: Manual
Change-Id: I4ef6433391c8758626334731d2b5de038e4468ae
Merged-In: I4ef6433391c8758626334731d2b5de038e4468ae
(cherry picked from I4ef6433391c8758626334731d2b5de038e4468ae)