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

cc/cc.go

@@ -163,7 +163,6 @@ type Flags struct {
 	GroupStaticLibs bool
 
 	proto            android.ProtoFlags
-	protoDeps        android.Paths
 	protoC           bool // Whether to use C instead of C++
 	protoOptionsFile bool // Whether to look for a .options file next to the .proto
 }
@@ -1594,6 +1593,10 @@ func (c *Module) depsToPaths(ctx android.ModuleContext) PathDeps {
 			return
 		}
 
+		if depTag == android.ProtoPluginDepTag {
+			return
+		}
+
 		if dep.Target().Os != ctx.Os() {
 			ctx.ModuleErrorf("OS mismatch between %q and %q", ctx.ModuleName(), depName)
 			return