* We modify connection management such that we now use more specific connection
types for {content, non-content, socket process} connections.
* We attach a native counterpart to the `GeckoProcessManager.ConnectionManager`
instance that listens for app foreground, app background, and when the
socket process is enabled, network state events.
* On app background, all non-content processes are assigned BACKGROUND priority.
Even though backgrounding the app will cause Android to drop all child
processes' priority regardless of our priority settings, we still do this as
to indicate to Android that these processes are relatively less important
than our parent process.
* When the socket process is enabled, we drop its priority when we detect that
we have no network connectivity. Note that the network management code does
the Right Thing if network connectivity changes while our app was in the
background: we receive the network state change event once we return to
foreground, therefore we do not need to do any special handling ourselves.
Differential Revision: https://phabricator.services.mozilla.com/D74478
