Workaround for Android N2 preview bug with EXTRA_RETURN_RESULT

A workaround for the bug discussed in https://gitlab.com/fdroid/fdroidclient/issues/631 The check against N is ugly, because: https://commonsware.com/blog/2016/03/17/backwards-compatibility-n-developer-preview.html

After removing EXTRA_RETURN_RESULT for Android N, we no longer get a result back on N, so instead of a proper error handling I always assume that the installation succeeds. Better than nothing...