Add WinPhone SignInDialog sample.

Accomodate to latest API changes.
Reference 10.2.4
Updated WinPhone SignInDialog sample
Accommodate to ChallengeHandler API changes
SignInDialog sample renamed SignInChallengeHandlerSample
Merge branch 'dom/TestWinPhoneSignInDialog'

Conflicts:
	src/Common/Esri.ArcGISRuntime.Toolkit.TestApp/Internal/TOCControl.xaml.cs
	src/Common/Esri.ArcGISRuntime.Toolkit.TestApp/Samples/AttributionSample.xaml.cs
	src/Common/Esri.ArcGISRuntime.Toolkit.TestApp/Samples/LegendSample.xaml.cs
	src/Common/Esri.ArcGISRuntime.Toolkit.TestApp/Samples/TemplatePickerSample.xaml.cs
	src/WinPhone/Esri.ArcGISRuntime.Toolkit.TestApp/Esri.ArcGISRuntime.Toolkit.TestApp.WindowsPhone.csproj
	src/WinStore/Esri.ArcGISRuntime.Toolkit.TestApp/Esri.ArcGISRuntime.Toolkit.TestApp.WindowsStore.csproj