You need to update the default Android SDK proguard.jar with the latest version of Proguard found here:
https://sourceforge.net/projects/proguard/files/
I would recommend that you install this on the side of the default version that Android ships in android-sdk\tools\proguard. Simply rename the existing folder to something else and add the new version of proguard.
This is listed in the Xamarin.Android 7.0 release notes:
https://developer.xamarin.com/releases/android/xamarin.android_7/xamarin.android_7.0/
In which the following bug was logged as well:
https://bugzilla.xamarin.com/show_bug.cgi?id=44187
Which is coordinated with a Pull Request in the Xamarin.Android repository:
https://github.com/xamarin/xamarin-android/pull/209