gn2bp: Add rule to replace arg by $(out)

To support //base:build_date

Test: ./update_result.sh, m cronet_aml_base_build_date
Change-Id: Ieacf0d47738f2ca32b3bec5415ac95b02b39c322
diff --git a/tools/gn2bp/Android.bp.swp b/tools/gn2bp/Android.bp.swp
index aa9015a..938d492 100644
--- a/tools/gn2bp/Android.bp.swp
+++ b/tools/gn2bp/Android.bp.swp
@@ -53,6 +53,18 @@
     ],
 }
 
+// GN: //base:build_date
+genrule {
+    name: "cronet_aml_base_build_date",
+    cmd: "$(location build/write_build_date_header.py) $(out) 1664686800",
+    out: [
+        "base/generated_build_date.h",
+    ],
+    tool_files: [
+        "build/write_build_date_header.py",
+    ],
+}
+
 // GN: //build:buildflag_header_h
 filegroup {
     name: "cronet_aml_build_buildflag_header_h",