Ensure a single running instance of edit monitor

This cl ensures at most 1 instance of edit monitor running from the
same binary by killing any existing instance before starting.

Specifically, When an edit monitor process starts, it will write a pidfile containing its pid and if such pidfile already exists (which means there's another instance there), it will read the pid contained in that pidfile and kill the corresponding process first.

Test: atst daemon_manager_test
bug: 365617369
Change-Id: I76954344df649aa79a6ef07ce55a62985decdb53