Add the basic edit monitor logic

The edit monitor uses the watchdog lib to montior any file changes
(create, modify, move, delete) under
a specific directory, create a edit log based on the file change event
and send the edit log to Sawmill through clearcut.

Test: Atest edit_monitor_test
Bug: 365617369
Change-Id: Ib6e04a43e4dd78ca58c7ccdef56fb4f6bda6e7a5