Check that androidSdkRootPath directory exists before using as sdkPath

Bug: 178496313
Change-Id: Ib85184e77572b75a242d55f572aa44629614555b

Author: cya-x

source/AndroidResolver/src/PlayServicesResolver.cs

@@ -842,7 +842,9 @@ public static string AndroidSdkRoot {
                 // Unity 2019.3 added AndroidExternalToolsSettings which contains the Android SDK
                 // path so try to use that first.
                 var androidSdkRootPath = UnityCompat.AndroidExternalToolsSettingsSdkRootPath;
-                if (!String.IsNullOrEmpty(androidSdkRootPath)) sdkPath = androidSdkRootPath;
+                if (!String.IsNullOrEmpty(androidSdkRootPath)) {
+                    if (Directory.Exists(androidSdkRootPath)) sdkPath = androidSdkRootPath;
+                }
 
                 // Unity 2019.x added installation of the Android SDK in the AndroidPlayer directory
                 // so fallback to searching for it there.