Improve Android client API

[atsakiridis]

We 're no longer depending on being compatible with Twilio, so let's take a good look on how to improve the API as best as we can. Also:

• Do we need capability tokens?

Need to brainstorm with team on that

Let's also keep notes of issues we encounter with with current API:

• RCConnection.setConnectionListener() isn't very intuitive, let's see if we can avoid it somehow
• Make sure RCConnection.local/remoteMediaType are actually intuitive and if so document them
• Right now RCConnection.parameters aren't used since we don't yet offer capabilities. But they are misleading, as we pass a set of parameters via RCDevice.connect(), which aren't kept anywhere within RCConnection
• Consider utilizing broadcast and broadcastReceivers to notify of SDK events, right now the whole thing is very rigid
• Calling RCClient.listDevices().get(0); to get a reference to RCDevice object in other activities (apart from the activity that initialized it) looks pretty bad. Need to improve that
• Not sure RCClient is needed as a means to initialize RCDevice. I have the impression it only adds complexity the way we use it. Since we're probably interested for only on RCDevice to be ever available I think we could get rid of it and have a logic more like what we have on iOS. In any case we need to consider this
• Some callbacks defined at RCConnectionListener & RCDeviceListener might not be needed in an App. Right now it is mandatory that the App implements them all which can be tedious. See if we can work around that.

[atsakiridis]

Moving this to next release as we did for iOS