616: Use new Google Auth API for signIn

[tasomaniac]

This PR uses the new Google Auth API.

• With this we removed couple deprecated methods/classes from Plus API.
• GoogleApiClientFactory with special ApiClient just for login.
• Removed all login related stuff from GdgActivity Login should not be done in the base class. It is now explicitly done by using the special ApiClient.
• Removed custom G+ login assets. Instead we are using the button from Google. Unfortunately recently, they removed all G+ branding. Now it is not possible to have a G+ button. It is only Google button.
• SharedPrefs is set only when the login is successful. In the past, we were setting it true when sign-in was clicked. Which means that the user wants login. And then it was handled automatically in GdgActivity. But now, we are handling it explicitly.

[tasomaniac]

I went ahead and merged this. Because future stuff will be depending on this.

I've tested it pretty heavily, it looks like it works very well. I will now push this to internal alpha build.