Deny rust warnings by default.

* "-D warnings" means "deny all warnings" and make them errors.
* Modules with warnings should fix all warnings or use
        deny_warnings: false

Bug: 141699953
Test: mm in projects with Rust modules
Change-Id: I6310dee8e34b7780937e8fc1834016a04a943a2f

rust/config/global.go

@@ -17,6 +17,8 @@ package config
 import (
 	"strings"
 
+	"github.com/google/blueprint/proptools"
+
 	"android/soong/android"
 	_ "android/soong/cc/config"
 )
@@ -33,6 +35,8 @@ var (
 		"libtest",
 	}
 
+	DefaultDenyWarnings = proptools.BoolPtr(true)
+
 	deviceGlobalRustFlags = []string{}
 
 	deviceGlobalLinkFlags = []string{