Initial cleanup for plugin extensions

[nwhite-riot]

In order for platform-specific plugin extensions to work, a few changes needed to be made:

• Replace explicit #ifdef PLATFORM_[NAME] checks and replace with COMPILED_PLATFORM_HEADER for subsystem (but more will be done in the future)
• Replace explicit platform enable/disable bool values with array of strings
  • Coupled with a bool toggle to enable for all supported platforms, but maintain backwards compatibility by seeding the array with previous values
• Renamed Apple and Android subsystem files to match Unreal's expected format (so that COMPILED_PLATFORM_HEADER will work)
  • Linux and Windows were already done as part of Refactor code to better align with Unreal's structure #745
• Update Microsoft/Windows subsystems in preparation for Xbox plugin extension