[Xamarin.Android.Tools.AndroidSdk] Downgrade build-tools to API-30

Context: https://developer.android.com/studio/releases/build-tools
Context: https://dl-ssl.google.com/android/repository/repository2-1.xml

Commit 440e6be bumped `$(AndroidSdkBuildToolsVersion)` to 31.0.0,
which seemed reasonable at the time…until we tried to actually build
xamarin-android against build-tools 31.0.0, which promptly failed:

	COMPILETODALVIK : error : Unable to access jarfile /Users/runner/Library/Android/sdk/build-tools/31.0.0/lib/dx.jar

Turns Out™ that the Android SDK Build-tools package dropped `dx.jar`:

	% curl -o repo.xml https://dl-ssl.google.com/android/repository/repository2-1.xml

	% xpath -q -e '//remotePackage[@path="build-tools;31.0.0"]/archives/archive[host-os="macosx"]/complete/url' repo.xml
	<url>d32e21a8aa8492ef8b86a489f601da425842b5da.build-tools_r31-macosx.zip</url>

	% curl -o x.zip https://dl-ssl.google.com/android/repository/d32e21a8aa8492ef8b86a489f601da425842b5da.build-tools_r31-macosx.zip

	% unzip -l x.zip | grep dx.jar
	# no match

There's (currently) no mention of this removal at the Android
[SDK Build Tools release notes][0] page, nor is there an explicit
mention that `dx` is *obsolete*…

Regardless, Xamarin.Android still supports the use of `dx`, and
doesn't generate any warning messages about it being obsolete.
Thus, in the meantime, continue to use/prefer
`$(AndroidSdkBuildToolsVersion)`=30.0.3, which *does* contain `dx.jar`.

[0]: https://web.archive.org/web/20210608191407/https://developer.android.com/studio/releases/build-tools

Author: jonpryor

src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.Versions.props

@@ -9,7 +9,7 @@
     If this file is changed the submodule for androidtools should be updated, 
     along with any other repo which references androidtools. 
     -->
-    <AndroidSdkBuildToolsVersion Condition="'$(AndroidSdkBuildToolsVersion)' == ''">31.0.0</AndroidSdkBuildToolsVersion>
+    <AndroidSdkBuildToolsVersion Condition="'$(AndroidSdkBuildToolsVersion)' == ''">30.0.3</AndroidSdkBuildToolsVersion>
     <AndroidCommandLineToolsVersion Condition=" '$(AndroidCommandLineToolsVersion)' == '' ">5.0</AndroidCommandLineToolsVersion>
     <AndroidSdkPlatformToolsVersion Condition="'$(AndroidSdkPlatformToolsVersion)' == ''">31.0.3</AndroidSdkPlatformToolsVersion>
     <AndroidSdkEmulatorVersion Condition="'$(AndroidSdkEmulatorVersion)' == ''"></AndroidSdkEmulatorVersion>