Fixing build constraints for GAE and GAE managed runtime.

GAE managed VM runtime contains both appengine and appenginevm
build tags. Currently, appengine.go is being built even if user
wants to use the package on managed VM that errors with no
"appengine" package to import.

google/appengine.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build appengine
+// +build !appenginevm
 
 package google
 

google/appenginevm.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !appengine
+// +build appenginevm
 
 package google