[release/8.0.2xx] [Xamarin.Android.Build.Tasks] DTBs should not rm generator output

[jonathanpeppers]

Backport of: #8706

Fixes: #8658
Fixes: #8698

Design-time builds don't play nicely with binding project builds:

% dotnet new androidlib
% cat > Example.java <<EOF
package e;

public class Example {
    public static void e() {
    }
}
EOF
% dotnet build -p:DesignTimeBuild=true -v:diag

After this initial Design-Time build, we have the following generated source code for the binding:

% find obj -iname \*.cs | xargs ls -l
-rw-r--r--  1 user staff   197 Mar 25 19:22 obj/Debug/net8.0-android/.NETCoreApp,Version=v8.0.AssemblyAttributes.cs
-rw-r--r--  1 user staff   441 Mar 25 19:22 obj/Debug/net8.0-android/__Microsoft.Android.Resource.Designer.cs
-rw-r--r--  1 user staff  2975 Mar 25 19:22 obj/Debug/net8.0-android/generated/src/E.Example.cs
-rw-r--r--  1 user staff  1518 Mar 25 19:22 obj/Debug/net8.0-android/generated/src/Java.Interop.__TypeRegistrations.cs
-rw-r--r--  1 user staff   696 Mar 25 19:22 obj/Debug/net8.0-android/generated/src/__NamespaceMapping__.cs
-rw-r--r--  1 user staff  1094 Mar 25 19:22 obj/Debug/net8.0-android/gxa-8706.AssemblyInfo.cs
-rw-r--r--  1 user staff   407 Mar 25 19:22 obj/Debug/net8.0-android/gxa-8706.GlobalUsings.g.cs

Run a Design-Time build again:

% dotnet build -p:DesignTimeBuild=true -v:diag

…and we're now missing files (?!):

% find obj -iname \*.cs | xargs ls -l
-rw-r--r--  1 user staff   197 Mar 25 19:22 obj/Debug/net8.0-android/.NETCoreApp,Version=v8.0.AssemblyAttributes.cs
-rw-r--r--  1 user staff   441 Mar 25 19:22 obj/Debug/net8.0-android/__Microsoft.Android.Resource.Designer.cs
-rw-r--r--  1 user staff  1094 Mar 25 19:22 obj/Debug/net8.0-android/gxa-8706.AssemblyInfo.cs
-rw-r--r--  1 user staff   407 Mar 25 19:22 obj/Debug/net8.0-android/gxa-8706.GlobalUsings.g.cs

In particular, $(IntermediateOutputPath)generated/*/**.cs is gone, including E.Example.cs!

The result of this is that Design-Time builds and "normal" builds "fight" each other, constantly generating and deleting files, slowing down incremental builds.

The root of 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 it was running during a design time build! During design time builds the binding library item groups are not evaluated, so the _ClearGeneratedManagedBindings target would run, deleting everything.

Fix this by ensuring we only run the _ClearGeneratedManagedBindings target in in "standard"/non-Design-Time builds.

[jonathanpeppers]

There are a couple test failures, we can ignore:

APK tests have a few:

System.Net.Http.HttpRequestException : net_http_message_not_success_statuscode_reason, 404, Not Found

Then 1 MSBuild test:

Exceeded expected time of 13500ms, actual 21371.052ms

But when I download the .binlog it's not any of our tasks:

There seems to be 10 lost seconds in MSBuild somewhere?

Going to just ignore, might be general slowness on the build machine.