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

android/Android.bp

@@ -17,6 +17,8 @@ bootstrap_go_package {
         "soong-response",
         "soong-shared",
         "soong-ui-metrics_proto",
+        "golang-protobuf-proto",
+        "golang-protobuf-encoding-prototext",
     ],
     srcs: [
         "androidmk.go",