lmkd: Prevent non-main threads being registered or killed by lmkd

Only thread group leaders should be registered with lmkd. Add a check to
ignore any non-leader TIDs and generate an error if such condition is
detected. Run the same check before killing a process to detect cases of
non-leader TIDs being used to kill a process. This might happen if PIDs
overflow and previously registered PID gets reused for a non-leader
thread in the following scenario:

1. pid X is a thread group leader and is registered with lmkd
2. pid X dies without lmkd knowing it and pid gets recycled
3. process Y creates a thread with tid X
4. lmkd kills pid X which results in process Y being killed

Bug: 136408020
Test: lmkd_unit_test
Change-Id: I46c5a0b273f2b72cefc20ec59b80b4393f2a1a37
Signed-off-by: Suren Baghdasaryan <surenb@google.com>