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/context.go b/ui/build/context.go
index 43e1e0f..f5e987e 100644
--- a/ui/build/context.go
+++ b/ui/build/context.go
@@ -20,7 +20,7 @@
 
 	"android/soong/ui/logger"
 	"android/soong/ui/metrics"
-	"android/soong/ui/metrics/metrics_proto"
+	soong_metrics_proto "android/soong/ui/metrics/metrics_proto"
 	"android/soong/ui/status"
 	"android/soong/ui/tracer"
 )