Remove explicit SDK version

See react-native-async-storage/async-storage#810

Update SDK logic based on PR feedback

Author: chrisglein

windows/ReactNativePicker/ReactNativePicker.vcxproj

@@ -14,13 +14,16 @@
     <AppContainerApplication>true</AppContainerApplication>
     <ApplicationType>Windows Store</ApplicationType>
     <ApplicationTypeRevision>10.0</ApplicationTypeRevision>
-    <WindowsTargetPlatformVersion Condition=" '$(WindowsTargetPlatformVersion)' == '' ">10.0.18362.0</WindowsTargetPlatformVersion>
-    <WindowsTargetPlatformMinVersion>10.0.17763.0</WindowsTargetPlatformMinVersion>
   </PropertyGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
   <PropertyGroup Label="ReactNativeWindowsProps">
     <ReactNativeWindowsDir Condition="'$(ReactNativeWindowsDir)' == ''">$([MSBuild]::GetDirectoryNameOfFileAbove($(SolutionDir), 'node_modules\react-native-windows\package.json'))\node_modules\react-native-windows\</ReactNativeWindowsDir>
   </PropertyGroup>
+  <Import Project="$(ReactNativeWindowsDir)\PropertySheets\External\Microsoft.ReactNative.WindowsSdk.Default.props" Condition="Exists('$(ReactNativeWindowsDir)\PropertySheets\External\Microsoft.ReactNative.WindowsSdk.Default.props')" />
+  <PropertyGroup Label="Fallback Windows SDK Versions">
+   <WindowsTargetPlatformVersion Condition=" '$(WindowsTargetPlatformVersion)' == '' ">10.0.18362.0</WindowsTargetPlatformVersion>
+   <WindowsTargetPlatformMinVersion Condition=" '$(WindowsTargetPlatformMinVersion)' == '' ">10.0.17763.0</WindowsTargetPlatformMinVersion>
+  </PropertyGroup>
   <ItemGroup Label="ProjectConfigurations">
     <ProjectConfiguration Include="Debug|ARM">
       <Configuration>Debug</Configuration>