Handle RESULT_CANCELED at onActivityResult to detect user cancel

[qua-iy]

On android, if I cancel sign in dialog by tapping outside of the box, it will return SignInException with status of Error. This PR will fix this issue and it will return status of Cancel.
I think this is related to #53.

[violet7]

Wait for SignInFragment appear, minimize app, tap app icon, crash.
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.android.gms.auth.api.signin.GoogleSignInAccount com.google.android.gms.auth.api.signin.GoogleSignInResult.getSignInAccount()' on a null object reference

[qua-iy]

@violet7 Hi, I think your issue exists originally, because GoogleSignInResult.getSignInAccount() I did not changed place of calling or condition. I maybe fixed it, but I thinks it should be fixed in another PR.

[leongkee]

Hi. I am using the Google Sign-in Plugin from google-signin-plugin-1.0.4.unitypackage in the Unity project. The package is downloaded from https://github.com/googlesamples/google-signin-unity/releases/tag/v1.0.4.

It still give me error when I cancel the sign-in.
How could I get the version that has fixed this issue and will return the status of "Cancel"?

How do I merge this fix into the Unity project?
How do I modify the GoogleSignInFragment.java file?

Please advise. Thanks.