Android compiler warnings

I'm finally getting this android app into my already existent iOS/PCL solution. Getting 103 warnings like so:
Warning 65 android\view\ActionMode$Callback.class(android\view:ActionMode$Callback.class): major version 51 is newer than 50, the highest major version supported by this compiler.

All of them reference a different class, but all say major version 51 is newer than 50...

I have installed all updates using the SDK manager in VS. Any ideas much appreciated.

Thanks!