Fix listening stats not sending the device id

[geekygecko]

This change fixes the listening stats on Android, in the current release any new Android stats aren't being added to the user's summary.

When I removed adding the device ID as part of another fix I broke the syncing of stats. The User Sync API requires the unique device ID (a random UUID). It's used to store the stats for the current device and will keep overwriting the stats values for this device. As the device ID was no longer being sent, the device ID was empty in the database. Empty device IDs are ignored when getting the stats summary from the server, so it's the same as not sending them to the server.

Fixes #238

Test Steps

1. Logout of your account and clear the apps data.
2. Create a new account.
3. Play an episode to create some listening stats.
4. On the Profile tab, tap the "Refresh now" button to manually sync the new stats.
5. Login to the staging Web Player and view the stats https://play.pocketcasts.net/stats

If this is working correctly the stats from Android should be appearing.

[ashiagr]

Stats matched on the web player and android app. Thanks for the fix!