Bump Android SDK/build tools version to 31 (#485)

diff --git a/build/config/android/config.gni b/build/config/android/config.gni
index f720442..1f2181b 100644
--- a/build/config/android/config.gni
+++ b/build/config/android/config.gni
@@ -16,8 +16,8 @@
 
   if (!defined(default_android_sdk_root)) {
     default_android_sdk_root = "//third_party/android_tools/sdk"
-    default_android_sdk_version = "30"
-    default_android_sdk_build_tools_version = "30.0.2"
+    default_android_sdk_version = "31"
+    default_android_sdk_build_tools_version = "31.0.0"
   }
 
   declare_args() {