Preparing a project for a platform causes changes for the other platform

[Mitko-Kerezov]

If one adds and prepares platform A in a fresh project, then adds and prepares platform B and then runs prepare for A again, CLI wrongfully registers changes for platform A and prepares anew, even though platform A and B have nothing in common.

Steps to reproduce:

• tns create prepareTest && cd prepareTest
• tns prepare android
• tns prepare ios
• tns prepare android

Expected result: Skip preparing for android
Actual result: Project is prepared for the android platform

Note: This affects tns run directly too. If one should run tns run android followed by tns run ios and another tns run android the last run command will actually unnecessarily prepare and build the project.