Make javanano a plugin

Treat proto.type: "javanano" as a plugin by explicitly passing the
path to protoc-gen-javanano.  This allows removing the javanano
customizations from the core plugin code, which no longer supports
javanano natively.

Bug: 117607748
Test: m checkbuild
Change-Id: Id7ed92d45583a7930f35c6734605d9304704bdbc
Merged-In: Id7ed92d45583a7930f35c6734605d9304704bdbc
diff --git a/java/proto.go b/java/proto.go
index 22a3eed..f5c233c 100644
--- a/java/proto.go
+++ b/java/proto.go
@@ -82,6 +82,7 @@
 			typeToPlugin = "javamicro"
 		case "nano":
 			flags.proto.OutTypeFlag = "--javanano_out"
+			typeToPlugin = "javanano"
 		case "lite":
 			flags.proto.OutTypeFlag = "--java_out"
 			flags.proto.OutParams = append(flags.proto.OutParams, "lite")