NeuralNetworks: 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: Ief00a52c59aba17da6f9930c000bdd1d58d4b588

Android.bp

diff --git a/Android.bp b/Android.bp
index 56e7903..744c7c6 100644
--- a/Android.bp
+++ b/Android.bp
@@ -37,6 +37,7 @@
 
 cc_defaults {
     name: "neuralnetworks_defaults",
+    cpp_std: "gnu++17",
     cflags: [
         "-O3",
         "-Wall",