[build] Import $(MSBuildProjectDirectory).override.props

[jonpryor]

Commit b6431ac introduced support for Configuration.Override.props,
which eventually allowed overriding the external/xamarin-android-tools
path via the $(XamarinAndroidToolsDirectory) MSBuild property in
commit 32f0c9c.

A problem with this approach is that it requires that the host repo
create the Java.Interop/Configuration.Override.props file before
running make -C …/Java.Interop prepare.

This can be simplified: update Directory.Build.props so that it
will also <Import/> the file
$(MSBuildProjectDirectory).override.props.
$(MSBuildProjectDirectory) is:

The absolute path of the directory where the project file is
located, for example C:\MyCompany\MyProduct.

Do not include the final backslash on this property.

The "Do not" is grammatically odd, as it's set by MSBuild.
It does not include a final backslash.

This means that we can place the .override.props file in the
directory containing the Java.Interop checkout, with a name that
matches the git submodule directory name:

external/my-Java.Interop-checkout                   # git submodule to xamarin/Java.Interop
external/my-Java.Interop-checkout.override.props    # now automatically imported