lmkd: Implement pid purge command to clear old pids when zygote restarts

lmkd keeps a list of pids registered by ActivityManager, however on rare
occasions when framework restarts and lmkd survives that list has to be
purged. Implement a command that can be used to clear the pid list.

Bug: 116801366
Test: locally by killing zygote process
Change-Id: I71d6012f86bb83a73edd5b687e05a0848e0569b1
Signed-off-by: Suren Baghdasaryan <surenb@google.com>