Update build processes

[StephenHodgson]

XRTK - Mixed Reality Toolkit Feature Request

Is your feature request related to a problem? Please describe

Especially for Android, it has become more apparent that we need to better handle post build steps that write to the Android manifest. But we'd also like to support building for a specific platform using the command line as well for our CI/CD pipeline.

Core Changes

At the moment, we kind of lump everything together, but we need to do something where each platform can extend the build window as needed to inject their platform specific options.

Platform Project Changes

Each platform will need to implement and extend the build window options and have a way to switch to the correct build platform target, if necessary, and do any pre/post build steps