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/cc/compiler.go b/cc/compiler.go
index fe46a3c..f9af4d8 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -221,6 +221,7 @@
 	deps.GeneratedSources = append(deps.GeneratedSources, compiler.Properties.Generated_sources...)
 	deps.GeneratedHeaders = append(deps.GeneratedHeaders, compiler.Properties.Generated_headers...)
 
+	android.ProtoDeps(ctx, &compiler.Proto)
 	if compiler.hasSrcExt(".proto") {
 		deps = protoDeps(ctx, deps, &compiler.Proto, Bool(compiler.Properties.Proto.Static))
 	}