Bump to NDK r19c (#2860)

Upstream [changes][0]:

  * [Issue 912][1]: Prevent the CMake toolchain file from clobbering
    a user specified `CMAKE_FIND_ROOT_PATH`.
  * [Issue 920][2]: Fix clang wrapper scripts on Windows.

[0]: https://github.com/android-ndk/ndk/wiki/Changelog-r19#r19c
[1]: android/ndk#912
[2]: android/ndk#920

build-tools/scripts/Ndk.targets

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
-    <AndroidNdkVersion Condition=" '$(AndroidNdkVersion)' == '' ">19b</AndroidNdkVersion>
+    <AndroidNdkVersion Condition=" '$(AndroidNdkVersion)' == '' ">19c</AndroidNdkVersion>
     <AndroidNdkApiLevel_ArmV7a Condition=" '$(AndroidNdkApiLevel_ArmV7a)' == '' ">16</AndroidNdkApiLevel_ArmV7a>
     <AndroidNdkApiLevel_ArmV8a Condition=" '$(AndroidNdkApiLevel_ArmV8a)' == '' ">21</AndroidNdkApiLevel_ArmV8a>
     <AndroidNdkApiLevel_X86 Condition=" '$(AndroidNdkApiLevel_X86)' == '' ">16</AndroidNdkApiLevel_X86>