ActivityManager: Add dumpsys command to report number of lmkd kills

Add support for "dumpsys activity lmk" command to report the number
of lmk kills. This change also enables two-way communicatoin between
ActivityManager and lmkd, introduces new LMK_GETKILLCNT command in lmkd
communication protocol and makes writeLmkd function synchronized to
prevent races.

Usage example:
#adb shell dumpsys activity lmk
ACTIVITY MANAGER LMK KILLS (dumpsys activity lmk)
  Total number of kills: 55
    kills at or below oom_adj 906: 55
    kills at or below oom_adj 900: 40
    kills at or below oom_adj 800: 38
    kills at or below oom_adj 700: 22
    kills at or below oom_adj 600: 19
    kills at or below oom_adj 500: 18
    kills at or below oom_adj 400: 13
    kills at or below oom_adj 300: 12
    kills at or below oom_adj 200: 10
    kills at or below oom_adj 100: 8
    kills at or below oom_adj 0: 1

Bug: 117126077
Test: used lmkd_unit_test to verify correct reporting
Change-Id: I71671b59040d670655171dacf4e7615b76398aab
Signed-off-by: Suren Baghdasaryan <surenb@google.com>