Context: dotnet#2247
Context: dotnet#2128
The `_CopyIntermediateAssemblies` target has a bug for incremental
builds, reproduced by the following:
- Build a Xamarin.Android app project
- `touch` the project's assembly in `$(IntermediateOutputPath)`
- Build a second time. `_CopyIntermediateAssemblies` will run as
  expected.
- Build a third time. `_CopyIntermediateAssemblies` will still run!
When in this state, `_CopyIntermediateAssemblies` will always run.
This is bad because it triggers other expensive targets like
`_UpdateAndroidResgen` _every time_.
I believe I introduced this in dotnet#2128, which was when I saw
`_CopyIntermediateAssemblies` taking more time than it used to. It was
a good tradeoff though, since it prevented `_UpdateAndroidResgen` from
running too often. 15.9 does not have dotnet#2128.
To fix this problem properly, I introduced a new
`_copyintermediate.stamp` file to be used as the `Outputs` of the
target. In addition to fixing our incremental build here, there should
be performance gains in only verifying the timestamp of one file in
`Outputs`.
The `Inputs` of the `_CopyIntermediateAssemblies` were also incorrect,
as it was not using the "satellite" assembly files as an input.
This does not fully complete dotnet#2247, as there are two other targets
listed I need to investigate further. These are likely unrelated to
the changes in dotnet#2128.