codec2 (mainline): pin cpp_std to gnu++17

This project doesn't build with gnu++20 and prevents AOSP default to be
upgraded to a newer standard. Once the build failure is fixed here, this
change can be reverted.

Bug: 311052584
Test: m checkbuild
Change-Id: Ie79a03959c6eb1aca7e9de95e06529f3d874de1d
diff --git a/media/codec2/vndk/Android.bp b/media/codec2/vndk/Android.bp
index bbe228c..265d87d 100644
--- a/media/codec2/vndk/Android.bp
+++ b/media/codec2/vndk/Android.bp
@@ -171,6 +171,7 @@
 // public dependency for implementing Codec 2 components
 cc_defaults {
     name: "libcodec2-impl-defaults",
+    cpp_std: "gnu++17",
 
     shared_libs: [
         "libbase", // for C2_LOG