My personal experience with C++ on Windows (CE in 2002) was not very encouraging WRT portability. You may get more commonality between destop Windows and WP7 than between WP7 and Android but the code may end up with a lot of ifdefs and windows-isms in it. It's been a while, so YMMV.

It could, in fact, be easier to mechanically translate between Java and C# than going C++.