This PR fixes some issues with connections and threading. Specifically, the change in #618 introduced threading in `HostConnection`, which had some issues. To fix them, the following changes were made:
- A specific TCP listener thread is used and manually started, instead of using one of the threads in the pool. The TCP listener thread is a long lived thread that should always be running (as long as the connection is through TCP), blocked listening on the TCP socket, so it shouldn't be managed in a pool, where, theoretically, it can be paused and reused.
- Changed the number of threads to 5. We shouldn't need more than this, otherwise we can overwhelm some Kodi hardware.
- Had to sprinkle some `synchronized` to avoid race conditions. For instance, through a TCP connection, as soon as Kore is opened (on the remote screen) it will call at least `GetActivePlayers`, `GetProperties`, `Ping`. If the TCP socket isn't set up yet, each of these calls would create a socket (and a TCP listener thread), so we would open 3 or more sockets when we should just open 1. A `synchronized` in `executeThroughTcp` prevents this. The others prevent similar issues.
Aditionally:
- Tweaked the playlist fetching code, so that it happens exclusively in `HostConnectionObserver` and when PlaylistFragment is notified of changes, it already gets the playlist data. This somewhat simplifies the code, and makes it more consistent with the Player Observer code;
- Change `EventServerConnection` to accept a Handler on which to post the result of the connection, so that the caller can control on which thread the result is called;
- Calls to the various `RegisterObserver` loose the reply immediately parameter, as it was always true.
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