[Xamarin.Android.Build.Tasks] set $(AndroidSdkDirectory) if blank (#2225)

In d6009ef, I fixed an issue when `$(AndroidSdkDirectory)` is set to
a non-existent directory.

However, downstream in `monodroid`, there is a test project with an
MSBuild target such as:

	<Exec Command="&quot;$(JavaSdkDirectory)\bin\javac&quot; -etc" />

`/p:JavaSdkDirectory` is not passed in here at the command line or
imported, causing `/bin/javac` is attempted to be used, which doesn't
work, as it doesn't exist.

This brings up an important point.

`$(AndroidSdkDirectory)`, `$(AndroidNdkDirectory)` and
`$(JavaSdkDirectory)` were public properties that were set if blank,
So we can't just remove them!  There could be unknown projects, NuGet
packages, etc. relying on them.

I think the fix here is to set these properties if they are blank.
Internally in xamarin-android's MSBuild targets, we should always use
the private underscore-prefixed versions such as
`$(_AndroidSdkDirectory)`.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -685,6 +685,9 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 			AndroidNdkPath="$(AndroidNdkDirectory)"
 			JavaSdkPath="$(JavaSdkDirectory)"
 			ReferenceAssemblyPaths="$(_XATargetFrameworkDirectories)">
+		<Output TaskParameter="AndroidNdkPath"            PropertyName="AndroidNdkDirectory" Condition=" '$(AndroidNdkDirectory)' == '' " />
+		<Output TaskParameter="AndroidSdkPath"            PropertyName="AndroidSdkDirectory" Condition=" '$(AndroidSdkDirectory)' == '' " />
+		<Output TaskParameter="JavaSdkPath"               PropertyName="JavaSdkDirectory"    Condition=" '$(JavaSdkDirectory)' == '' " />
 		<Output TaskParameter="AndroidNdkPath"            PropertyName="_AndroidNdkDirectory" />
 		<Output TaskParameter="AndroidSdkPath"            PropertyName="_AndroidSdkDirectory" />
 		<Output TaskParameter="JavaSdkPath"               PropertyName="_JavaSdkDirectory" />