[Java.Interop] Place JniEnvironment.g.cs in $(IntermediateOutputPath) to support parallel builds.

[jpobst]

Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=6496140

Due to multitargeting of Java.Interop.csproj and parallel builds (no -m:1), we can write the file Java.Interop/JniEnvironment.g.cs twice in a build:

// net7.0
Command = /Users/builder/azdo/_work/1/s/xamarin-android/bin/Release/dotnet/dotnet "/Users/builder/azdo/_work/1/s/xamarin-android/external/Java.Interop/bin/BuildRelease-net7.0/jnienv-gen.dll" Java.Interop/JniEnvironment.g.cs obj/Release/net7.0/jni.c

// netstandard2.0
Command = mono "/Users/builder/azdo/_work/1/s/xamarin-android/external/Java.Interop/bin/BuildRelease/jnienv-gen.exe" Java.Interop/JniEnvironment.g.cs obj/Release/netstandard2.0/jni.c

As a race condition, this can cause the generation invocations to clobber each other, or to re-generate the file while the other is trying to compile with it:

/Users/builder/azdo/_work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop/JniEnvironment.g.cs(6926,21): error CS1027: #endif directive expected [/Users/builder/azdo/_work/1/s/xamarin-android/external/Java.Interop/src/Java.Interop/Java.Interop.csproj]

Fix this by moving the file to $(IntermediateOutputPath) so that each can have their own copy.

[jonathanpeppers]

Reading 99f8990, we might not need the %(KeepDuplicates) anymore. I don't think the default C# wildcards would include it.

But we could also just leave this for now.

[jpobst]

Test XA run: dotnet/android#7226

There are test failures, but I'm betting any effects from this would break the build, not fail tests.