Add support for protoc plugins

Add a proto.plugin property to allow specifying a custom protoc
plugin to generate the code.

Fixes: 70706119
Test: m am StreamingProtoTest
Change-Id: I1ecdd346284b42bbcc8297019d98d2cd564eb94c
diff --git a/python/proto.go b/python/proto.go
index 813e91d..b3ffaa6 100644
--- a/python/proto.go
+++ b/python/proto.go
@@ -29,7 +29,7 @@
 	rule.Command().Text("rm -rf").Flag(outDir.String())
 	rule.Command().Text("mkdir -p").Flag(outDir.String())
 
-	android.ProtoRule(ctx, rule, protoFile, flags, nil, outDir, depFile, nil)
+	android.ProtoRule(ctx, rule, protoFile, flags, flags.Deps, outDir, depFile, nil)
 
 	// Proto generated python files have an unknown package name in the path, so package the entire output directory
 	// into a srcszip.