Rust: WriteFileRule instead of printf for protos

Use WriteFileRule instead of a rule that calls printf for generating the
module source file for rust_protobuf.

Bug: 171361369
Test: rust_protobuf modules compile
Change-Id: Ie83a30e19d7d09875f3d209662526783862d8ea0

rust/rust.go

@@ -260,7 +260,8 @@ type PathDeps struct {
 	CrtEnd   android.OptionalPath
 
 	// Paths to generated source files
-	SrcDeps android.Paths
+	SrcDeps          android.Paths
+	srcProviderFiles android.Paths
 }
 
 type RustLibraries []RustLibrary