Fix Player.cast not available before first source change

[MattiasBuelens]

In #34, we added some logic to handle the case where the Chromecast integration is attached after we construct a Player instance. However, this only checks if the integration is attached when the player's source changed. If you tried to use player.cast before the first source is set on the player, it would always return null.

Fix it by trying to update player.cast on every access. In the future, we should have the THEOplayer Android SDK handle this better, for example theoplayerView.cast could return dummy values (instead of null) if the Chromecast integration is not attached.

Fixes #47