Initialize accountManager on the main thread during migration
This fixes fallback from https://github.com/mozilla-mobile/android-components/pull/6395 in which FennecMigrator changed to accept a Lazy-wrapped reference to the account manager. This was done so that during regular (non-migrating) startup, a simple act of building a no-op migrator will not cause account manager initialization. However, during a migrating startup this creates a race to initialize account manager. Two participants in this race are: 1) FenixApplication running expensive operations (such as storage init, account manager init) after visual completeness. However, currently that just entails a 5 second delay from `onStart`. 2) FxA migration, which runs roughly at the mid-point of the migration. Account manager currently can't be initialized on a background thread (or it will crash). FxA migration runs on a background thread, so if (2) wins (that is, migrations before FxA migration take less than 5 seconds), then migration will crash. This patch adds an eager initialization right before we kick-off migrations but are still on the main thread, iff an FxA migration will be executed.