Fixes dotnet#8658
Fixes dotnet#8698

The PR fixes an issue where the bindings files in intermediate
`generated` folder get deleted on subsequent builds. This causes
the entire binding process to run again, slowing down builds.

The root fo the problem is the `_ClearGeneratedManagedBindings`
target. It was designed to clean out the `generated` folder in
the case where no binding libraries were present. However it
turns out if was running during a design time build! During
design time builds the binding library item groups are not
evaluated, so the `_ClearGeneratedManagedBindings` would run..
deleting everything.

To fix this issue we only run the `_ClearGeneratedManagedBindings`
in a standard build. The good thing is this allowed us time to
take a look at our incremental build process for bindings.
The binding generator does produce a `.projitems` file which lists
all the files it generated. We can use that data to incrementally
clean up the `generated` folder of old files. This way code that
is no longer needed can be removed. While we know the linker will
eventually remove unused classes and types, it is better to not
have to compile this stuff in the first place.

Context: https://github.com/xamarin/xamarin-android/pull/8706/files#r1538222261

dotnet@dd1f25b
("initial" commit) had code within the `<BindingsGenerator/>` task
to parse the `generator`-emitted `.projitems` file, to read out the
`@(Compile)` group:

    List<ITaskItem> files = new List<ITaskItem> ();
    var doc = XDocument.Load (GeneratedFileListFile);
    var compileItems = doc.XPathSelectElements ("//Project/ItemGroup/Compile");
    foreach (var item in compileItems) {
        var file = item.Attribute ("Include");
        if (file != null && File.Exists (file.Value)) {
            files.Add (new TaskItem (file.Value));
        }
    }
    GeneratedFiles = files.ToArray ();

There were two fundamental problems with this code:

 1. No XML namespaces
 2. The `File.Exists()` check.

`GeneratedFileListFile` would be the `.projitems` file,
which is a normal MSBuild file with a default xmlns:

    <?xml version="1.0" encoding="utf-8"?>
    <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
      <PropertyGroup>
        <DefineConstants>$(DefineConstants);ANDROID_1;ANDROID_2;ANDROID_3;ANDROID_4;ANDROID_5;ANDROID_6;ANDROID_7;ANDROID_8;ANDROID_9;ANDROID_10;ANDROID_11;ANDROID_12;ANDROID_13;ANDROID_14;ANDROID_15;ANDROID_16;ANDROID_17;ANDROID_18;ANDROID_19;ANDROID_20;ANDROID_21;ANDROID_22;ANDROID_23;ANDROID_24;ANDROID_25;ANDROID_26;ANDROID_27;ANDROID_28;ANDROID_29;ANDROID_30;ANDROID_31;ANDROID_32;ANDROID_33;ANDROID_34</DefineConstants>
      </PropertyGroup>
      <!-- Classes -->
      <ItemGroup>
        <Compile Include="$(MSBuildThisFileDirectory)E.Example.cs" />
        <Compile Include="$(MSBuildThisFileDirectory)Java.Interop.__TypeRegistrations.cs" />
        <Compile Include="$(MSBuildThisFileDirectory)__NamespaceMapping__.cs" />
      </ItemGroup>
      <!-- Enums -->
      <ItemGroup />
    </Project>

The original `doc.XPathSelectElements()` expression didn't have any
XML namespaces present, so `compileItems` would always be *empty*.

This can be fixed by using an `XmlNamespaceManager`:

	List<ITaskItem> files = new List<ITaskItem> ();
	Log.LogDebugMessage ($"# jonp: SHOULD process {GeneratedFileListFile}");
	var doc = XDocument.Load (GeneratedFileListFile);
	var nsmgr = new XmlNamespaceManager(new NameTable());
	nsmgr.AddNamespace ("msb", "http://schemas.microsoft.com/developer/msbuild/2003");
	var compileItems = doc.XPathSelectElements ("//msb:Project/msb:ItemGroup/msb:Compile", nsmgr);
	foreach (var item in compileItems) {
		var file = item.Attribute ("Include");
		if (file != null && File.Exists (file.Value)) {
			Log.LogDebugMessage ($"# jonp: found //Compile/@include value: {file.Value}");
			files.Add (new TaskItem (file.Value));
		}
	}
	GeneratedFiles = files.ToArray ();

Now `compileItems` has elements!

Then we encounter the `File.Exists()` problem: *we* are parsing the
`.projitems` file, ***not MSBuild***, so if (when) the file contains
MSBuild-isms such as `$(MSBuildThisFileDirectory)`, those pass through
unchanged!  For example, `compileItems.ElementAt(0)` would be:

    <Compile Include="$(MSBuildThisFileDirectory)E.Example.cs" />

`item.Attribute("Include").Value` would be:

    $(MSBuildThisFileDirectory)E.Example.cs

which means we're trying to *literally* do:

    File.Exists("$(MSBuildThisFileDirectory)E.Example.cs")

No expansion is occurring.

This has (approximately) ***no*** chance of working.

At minimum  we'd have to `string.Replace()` the sub-string
`$(MSBuildThisFileDirectory)` with `OutputDirectory`.

However, the fact that all the unit tests pass means one of two things:

 1. We need more unit tests ;-), or
 2. We don't actually need to parse the `.projitems` file.

Assume that (2) is the case: update `<BindingsGenerator/>` et al to
*remove* the processing of `.projitems`.

Does It Work™?