Implement a polyfill for PlatformApi for backward compatibility with 'old-style' platforms

To make the transition to new API smoother, the cordova-lib needs to expose PlatformApiPoly class according to PlatformApi spec, which will be used if platform doesn't provides its own PlatformApi.

Expected functionality:

• create/update platform
• execute platform's actions (build/run/add/update)
• do a prepare (needed for CLI workflow only)
• install/uninstall plugins