This commit adds two Glean pings to the Storage-Sync browser component:
one for history, and one for bookmarks. Both pings record the same set
of base metrics, including incoming and outgoing counts, sync duration,
the hashed FxA UID, and, most importantly, the failure reason if the
store fails to sync. The bookmarks ping records additional validation
data.

The Glean schema is a flattened version of the Sync ping that's
currently sent on Firefox Desktop and Firefox for iOS. Instead of
sending a single ping with multiple syncs, each having multiple
engines, we send one Glean ping per engine per sync.

The pings are recorded directly in the component, so any app that
consumes it should get pings for free.

Due to upgrading WorkManager dependency, testing of WorkManager now works so all of the tests depending on our workaround in `triggerWorkManager` needed a fix to prevent intermittent failures.  This required us to use the WorkManager TestDriver to trigger the conditions and now we test WorkManager as it is supposed to be tested.

This reverts commit b27b7c26.

Trying to initialize Glean on a process other than the main process results in an error when Glean tries to use WorkManager because WorkManager is only initialized on the main process.  This checks whether initialize is being called by the main process and returns if it isn't.

Reduce duplication by sharing logic for "sizes" in browser-icons
and "sizes" in WebAppManifest, which are supposed to be treated the same
in spec anyways.

This allows the slack:// (and other custom schemes).

This makes them easier to explain and reason about them.