iOS *.csproj <Import> is out of date

I switched to using the latest <Import> statements for the latest
Xamarin.iOS
issue #1920: Song always loops on Android
PremultiplyAlpha should be set true by default, DefaultValueAttribute does not set the value.
Merge pull request #1924 from danzel/mgcb-alpha

PremultiplyAlpha should be set true by default
Pass the current Platform/Profile down when building content. This allows content writers to save things different for different platforms.
Merge pull request #1925 from danzel/mgcb-platform

Pass the current Platform/Profile down when building content.
Merge pull request #1921 from dmanning23/AndroidSongFix

issue #1920: Song always loops on Android
Merge pull request #1915 from jonathanpeppers/develop

iOS *.csproj <Import> is out of date