Do not specify API level twice.

Change-Id: Ie9900610f44b290e971871a51036b5c51cb6be75
diff --git a/gn/standalone/BUILD.gn b/gn/standalone/BUILD.gn
index 16e391c..1866a79 100644
--- a/gn/standalone/BUILD.gn
+++ b/gn/standalone/BUILD.gn
@@ -127,7 +127,7 @@
       "-isystem$android_compile_sysroot/$android_compile_sysroot_subdir",
       "-isystem$android_compile_sysroot",
       "-DANDROID",
-      "-D__ANDROID_API__=21",
+      "-D__ANDROID_API__=${android_api_level}",
       "--target=$android_abi_target",
     ]
     cflags_cc += [