Upgrade to golang protobuf api v2

Major operations:
 * Fix the go_package entries to be consistent, as the new tool
   complains when it doesn't have a '/'.
 * Regenerate with the new protoc-gen-go tool.
 * github.com/golang/protobuf -> google.golang.org/protobuf
 * proto.[Un]MarshalText -> prototext.[Un]Marshal

Change-Id: Ie1147bd2457fafb66ba555461b3bf14f0561a25d
diff --git a/ui/build/config_test.go b/ui/build/config_test.go
index 7d4c76b..1f2158b 100644
--- a/ui/build/config_test.go
+++ b/ui/build/config_test.go
@@ -29,7 +29,7 @@
 	smpb "android/soong/ui/metrics/metrics_proto"
 	"android/soong/ui/status"
 
-	"github.com/golang/protobuf/proto"
+	"google.golang.org/protobuf/proto"
 )
 
 func testContext() Context {