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/genrule/genrule_test.go b/genrule/genrule_test.go
index 19b22f7..5cb51b8 100644
--- a/genrule/genrule_test.go
+++ b/genrule/genrule_test.go
@@ -527,4 +527,4 @@
 	return android.OptionalPathForPath(t.outputFile)
 }
 
-var _ HostToolProvider = (*testTool)(nil)
+var _ android.HostToolProvider = (*testTool)(nil)