Fix bug with changing cast app id to empty string
Fix bug with changing cast app id to empty string

If that method was called with an app id that was an empty
string, it would destroy the cast receiver, and then not make a
new one.
This lead to the ChromeCast callbacks all pointing to a destroyed
cast sender, which would then spew out errors whenever session
availability changed.
Normally, changing the app id to empty string doesn't make sense,
but it can happen if someone configures the UI thoughtlessly.
This changes the API callbacks provided to be static, and only
forward the calls to live senders.

Change-Id: I88593532791b3448be3ebdf7ef4df4d38c1bc6e4

tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player