The previous change had the fault of considering __UNIFIED__ANYTHING__ the same as __UNIFIED__ and if a user set that property, then he would lose the other (pointed out by jstedfast in the PR).
This commit fixes that.
In VS the property page for Build will show whatever is in DefineConstants, and then save it. Without this, that means that any time the Build page is saved, it duplicates the constants.
This fixes bug#32765: Bug 32765 - Conditional compilation symbols are duplicated everytime I reload the project or restart visual studio (https://bugzilla.xamarin.com/show_bug.cgi?id=32765)