Context: https://dev.azure.com/devdiv/DevDiv/_workitems/edit/1009374/

This is a first step toward making the Xamarin.Android.Build.Tasks
MSBuild error and warning messages localizable.  This first step covers
a few decisions:

  * Use the MSBuild message codes like `XA4214` as the names for the
    string resources.
  * When an MSBuild message code like XA4214 is associated with more
    than one string, append short descriptive phrases to the end of the
    code to create additional property names as needed.
  * Name the `.resx` file `Resources.resx`, and place it in the
    `src/Xamarin.Android.Build.Tasks/Properties/` directory, alongside
    the `AssemblyInfo.cs` file.  This follows what the nonpublic
    xamarin/designer and xamarin/XamarinVS repositories do.
  * Store the `.resx` file with LF line endings in the repo, but set
    `.resx` files to `text` mode in `.gitattributes` so the file can be
    checked out and edited with CRLF line endings on Windows.  The
    Visual Studio `.resx` resource editor uses CRLF line endings.
  * Use the `<Generator>ResXFileCodeGenerator</Generator>` Visual Studio
    IDE mechanism to generate a `Resources.Designer.cs` file that is
    explicitly included in the project as a `@(Compile)` item.  The
    `Resource.Designer.cs` file provides C# property names to access the
    `.resx` resources.  Visual Studio updates the file each time a
    change is saved to the corresponding `.resx` file.  This follows
    what the nonpublic xamarin/designer and xamarin/XamarinVS repos do.
    In the future, if `Xamarin.Android.Build.Tasks.csproj` is converted
    to a short-form project, it would probably be best to add a
    dependency on dotnet/arcade and switch to using the
    [`GenerateResxSource` mechanism][0] instead.  That would match what
    dotnet/sdk does.
  * Use the [comment style from dotnet/sdk][1] to provide a brief
    explanation of each format item like `{0}`, especially in cases
    where the expected contents of the format item might not be obvious
    from context.
  * Keep the MSBuild message codes as string literals in the `.cs`
    files.  This differs from what dotnet/sdk and microsoft/msbuild do.
    This might be incorrect.  It might be important to include the
    message code prefix in the string so that the localization team can
    decide on the appropriate capitalization after the `:` character.
    See for example the use of lowercase letters after the `:` character
    in the Italian translations in [microsoft/msbuild][2] and
    [dotnet/sdk][3].

String values from `.resx` resources are not subject to interpolation,
so any strings that use C# 6 string interpolation will need to be
replaced with format strings that use format items like `{0}`.  The
message for XA4214 did not use string interpolation, so no change was
needed for this first example.

This commit does not yet leverage the MSBuild tasks and targets from
[xliff-tasks][4] to generate satellite assemblies for localized string
resources.  That will be the next step.

This commit intentionally skips adding anything to
`Documentation/release-notes/` for now because these changes do not yet
affect anything interesting to end users.

[0]: https://github.com/dotnet/arcade/blob/e67d9f098029ebecedf11012a749b532d68ad2a9/Documentation/ArcadeSdk.md#generateresxsource-bool
[1]: https://github.com/dotnet/sdk/blob/9bab2f06387393a06a79e4234494eebf42f9cc10/src/Tasks/Common/Resources/Strings.resx#L349
[2]: https://github.com/microsoft/msbuild/blob/1f4524283e4e7f0dfae9a83557bf551c8a28cab9/src/Tasks/Resources/xlf/Strings.it.xlf
[3]: https://github.com/dotnet/sdk/blob/9bab2f06387393a06a79e4234494eebf42f9cc10/src/Tasks/Common/Resources/xlf/Strings.it.xlf
[4]: https://github.com/dotnet/xliff-tasks

Author: brendanzagaeski