[build] Properly set $(AndroidMxeInstallPrefix)

Commit a1799f48 [broke][0] the [build][1]:

	error : $(AndroidMxeFullPath) value of `/Users/builder/android-toolchain/mxe-a926b16` MUST end with `b9cbb53`!

[0]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/858/consoleText
[1]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/858/

Update `$(AndroidMxeInstallPrefix)` so that we can build.

Configuration.props

@@ -43,7 +43,7 @@
     <AndroidPreviousFrameworkVersion Condition=" '$(AndroidPreviousFrameworkVersion)' == '' ">v1.0</AndroidPreviousFrameworkVersion>
     <AndroidToolchainCacheDirectory Condition=" '$(AndroidToolchainCacheDirectory)' == '' ">$(HOME)\android-archives</AndroidToolchainCacheDirectory>
     <AndroidToolchainDirectory Condition=" '$(AndroidToolchainDirectory)' == '' ">$(HOME)\android-toolchain</AndroidToolchainDirectory>
-    <AndroidMxeInstallPrefix Condition=" '$(AndroidMxeInstallPrefix)' == '' And '$(NeedMxe)' == 'true' ">$(AndroidToolchainDirectory)\mxe-a926b16</AndroidMxeInstallPrefix>
+    <AndroidMxeInstallPrefix Condition=" '$(AndroidMxeInstallPrefix)' == '' And '$(NeedMxe)' == 'true' ">$(AndroidToolchainDirectory)\mxe-b9cbb53</AndroidMxeInstallPrefix>
     <AndroidMxeInstallPrefix Condition=" '$(HostOS)' == 'Linux' ">\usr</AndroidMxeInstallPrefix>
     <AndroidSdkDirectory Condition=" '$(AndroidSdkDirectory)' == '' ">$(AndroidToolchainDirectory)\sdk</AndroidSdkDirectory>
     <AndroidNdkDirectory Condition=" '$(AndroidNdkDirectory)' == '' ">$(AndroidToolchainDirectory)\ndk</AndroidNdkDirectory>