Bump to NDK r21c (#4744)

Context: https://github.com/android/ndk/wiki/Changelog-r21#r21c

Changes:

  * [Issue 1060][0]: A macOS app bundle that is signed and notarized is
    now available for download from our wiki and our website. Note that
    because only bundles may use RPATHs and pass notarization, the
    traditional NDK package for macOS *cannot* be notarized. The SDK
    will continue to use the traditional package as the app bundle
    requires layout changes that would make it incompatible with
    Android Studio.  The NDK is not quarantined when it is downloaded
    via the SDK manager, so is currently allowed by Gatekeeper. The SDK
    manager is currently the most reliable way to get the NDK for macOS.
  * [Issue 1207][1]: Fix fatal error in clang when building with -O2 on arm64.
  * [Issue 1239][2]: Fix network drive issues for clang.
  * [Issue 1229][3]: README.md turned back to ordinary file.

[0]: android/ndk#1060
[1]: android/ndk#1207
[2]: android/ndk#1239
[3]: android/ndk#1229

build-tools/xaprepare/xaprepare/ConfigAndData/BuildAndroidPlatforms.cs

@@ -5,8 +5,8 @@ namespace Xamarin.Android.Prepare
 {
 	class BuildAndroidPlatforms
 	{
-		public const string AndroidNdkVersion = "21b";
-		public const string AndroidNdkPkgRevision = "21.1.6352462";
+		public const string AndroidNdkVersion = "21c";
+		public const string AndroidNdkPkgRevision = "21.2.6472646";
 
 		public static readonly List<AndroidPlatform> AllPlatforms = new List<AndroidPlatform> {
 			new AndroidPlatform (apiName: "",                       apiLevel: 1,  platformID: "1"),