This patch switches the main logic from a boolean pref to an integer with different feature modes.
See nsIBounceTrackingProtection for the new modes supported. Dry-run mode has been merged into the mode
pref.
A new state is introduced MODE_ENABLED_STANDBY in which BTP still collects user activation signals, but
does not classify or purge bounce trackers. This new mode should be used when it's expected that the
feature may be toggled on/off after rollout. Collecting user activation even in the "disabled" state
ensures that when the feature gets re-enabled the user activation map is already warmed up and sites
important to the user are not classified + purged.
The BounceTrackingProtection singleton listens for changes on the mode pref and inits / tears down accordingly.
However it never shuts down fully. To fully disable the feature a restart is required.
Differential Revision: https://phabricator.services.mozilla.com/D217848
6d4789b415