SimpleAuth: Add support for Android Target SDK API 23 (currently Google Sign In crashes)

If you target API 23+ the app must prompt the user for GetAccounts permission:

https://blog.xamarin.com/requesting-runtime-permissions-in-android-marshmallow/

We need to add a call to CheckSelfPermission.

Without that, I am finding that Authenticate() throws an exception, which I catch, but even so my app terminates with:

System.InvalidOperationException: An attempt was made to transition a task to a final state when it had already completed.
  at System.Threading.Tasks.TaskCompletionSource`1[TResult].SetResult (TResult result) [0x0000c] in /Users/builder/data/lanes/4468/f913a78a/source/mono/mcs/class/referencesource/mscorlib/system/threading/Tasks/TaskCompletionSource.cs:322
  at Android.Gms.Extensions.GoogleApiClientExtensions+<BuildAndConnectAsync>c__AnonStorey0.<>m__0 (Android.OS.Bundle hint) [0x00000] in <1db671c6182d42a7b22c48ed6b33a584>:0
  at Android.Gms.Common.Apis.GoogleApiClientConnectionCallbacksImpl.OnConnected (Android.OS.Bundle bundle) [0x0000d] in <1db671c6182d42a7b22c48ed6b33a584>:0
  at Android.Gms.Common.Apis.GoogleApiClient+IConnectionCallbacksInvoker.n_OnConnected_Landroid_os_Bundle_ (System.IntPtr jnienv, System.IntPtr native__this, System.IntPtr native_connectionHint) [0x00011] in <1db671c6182d42a7b22c48ed6b33a584>:0
  at at (wrapper dynamic-method) System.Object:a6b03050-593a-4ddc-9bdf-441125fda0bf (intptr,intptr,intptr)