Fixed facebook/google provider strings

Changed from facebook and google to facebook.com and google.com respectively. This is needed for cross platform apps. These strings are now consistent with the [iOS](https://github.com/fullstackreact/react-native-firestack/blob/v3/ios/Firestack/FirestackAuth.m#L444) and [Web](https://firebase.google.com/docs/reference/js/firebase.auth.AuthCredential) code. Docs should be updated too. [Here's my pull request](https://github.com/fullstackreact/react-native-firestack/pull/284/files) for that.

Author: abcd-ca

android/src/main/java/io/fullstack/firestack/auth/FirestackAuth.java

@@ -159,9 +159,9 @@ public void onComplete(@NonNull Task<AuthResult> task) {
 
   @ReactMethod
   public void signInWithProvider(final String provider, final String authToken, final String authSecret, final Callback callback) {
-    if (provider.equals("facebook")) {
+    if (provider.equals("facebook.com")) {
       this.facebookLogin(authToken, callback);
-    } else if (provider.equals("google")) {
+    } else if (provider.equals("google.com")) {
       this.googleLogin(authToken, callback);
     } else
       // TODO