Bug 1758115 - Part 2: Streamline locking, initialization and shutdown for TimelineConsumers, r=smaug

The current implementation of TimelineConsumers contains some unnecessary
complexity due to how it is initialized as a singleton, and the need for it to
be initialized and used in a threadsafe way. This patch attempts to simplify it
by making all members static, and removing the need to explicitly observe
shutdown for cleanup.

In addition, this approach avoids the risk of the type being accessed from
off-main-thread during initialization or shutdown.

Depends on D150442

Differential Revision: https://phabricator.services.mozilla.com/D150443