Move collect_vpn_metrics flag to android_core_networking namespace

This change moves the collect_vpn_metrics aconfig flag from
services/core/java/com/android/server/connectivity/flags.aconfig
to core/java/android/net/flags.aconfig.

This relocation aligns the flag with the android_core_networking
namespace.

Bug: 306313287
Test: build
Flag: android.net.platform.flags.collect_vpn_metrics
Change-Id: I866623a4016b5835a9c0937900498b0fe5d172a8