Fix crash because onConnected is called multiple times

[SUPERCILEX]

Refactor mistake: I didn't notice that the old helper removed the listeners. I think that method is flawed though because if onConnectionFailed is called and later on onConnected also gets called, an IllegalStateException will still be thrown.

[samtstern]

This does seem functionally the same as unregistering the listeners as we're basically just ignoring follow-up calls, but can you add some comments above the trySet{Foo} calls explaining why we need to use that style?

[SUPERCILEX]

@samtstern Will do! The other reason I wanted to use a trySet is because extenders of GoogleApiHelper might depend on onConnected being called each time the client connects, not just the first time.

[SUPERCILEX]

@samtstern Done.