[ew] Simplify the entry watcher logic.
The responsibility of EntryWatcherService has been moved to WatcherManager
implementations.
Note, that the original code was unnecesarily complicated. After replacing
the observer interface with a simple callback, there is no need for an extra
service managing watchers anymore.
This was possible because with the new design entry watchers will no longer
survive reboots.
TEST=Compiles.
BUG=261491
Review URL: https://codereview.chromium.org/
642343004
Cr-Commit-Position: refs/heads/master@{#301319}