[d17-0][build] Allow Assembly "vendorization" (#136)

[BretJohnson]

Context: ff73f92
Context: 061bcc2
Context: https://github.com/xamarin/XamarinVS/pull/12550

Changing $(Version) with every commit is fun and all, but doesn't
solve all problems. Commit ff73f92 works "nicely" for MSBuild tasks
via <UsingTask/>. It doesn't work as well for "normal"
assembly references in a "normal" AppDomain context, because
assemblies are normally resolved via "assembly base name", not
the full path name including directory.

Thus, given Example.dll:

csc /out:Example.dll /r:Path/To/Xamarin.Android.Tools.AndroidSdk.dll

then when Example.dll is loaded, it will try to load
Xamarin.Android.Tools.AndroidSdk via a Load-by-name method, and
will load the first Xamarin.Android.Tools.AndroidSdk.dll loaded
into the AppDomain/AssemblyLoadContext, regardless of version.

There may not be a good way to control what that assembly is.

This causes grief with our peer IDE teams, as assembly versions are
still checked, but on mismatch an exception is thrown (!).

Commit 061bcc2 was an attempt to address this, but proved to be
incomplete.

Attempt to improve matters by introducing a "vendorization" protocol:

1. Update Directory.Build.props to import
   "parent directory.override.props", so that a "parent checkout"
   can easily override MSBuild properties.

2. Update the *.csproj files so that $(AssemblyName) is forced
   to start with $(VendorPrefix), and end with $(VendorSuffix).

This allows a parent checkout to set the $(VendorPrefix) and
$(VendorSuffix) properties, which will impact the assembly filenames
of all assemblies built in xamarin-android-tools.