hidl_test: 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: If854fb6d95f2133b593fc123d0bf568fb2cccd83
diff --git a/test/hidl_test/Android.bp b/test/hidl_test/Android.bp
index f5336e7..80d33a6 100644
--- a/test/hidl_test/Android.bp
+++ b/test/hidl_test/Android.bp
@@ -26,6 +26,8 @@
     name: "hidl_test-defaults",
     defaults: ["hidl-gen-defaults"],
 
+    cpp_std: "gnu++17",
+
     shared_libs: [
         "android.hidl.memory@1.0",
         "android.hidl.memory.token@1.0",