Validate and correct the VPN metrics

If there are invalid values in VPN metrics data, and this data is
uploaded to StatSD, then this garbage data will impact the
overall metrics in the system. Therefore, it's crucial to validate
and correct the VPN metrics before uploading.

Bug: 306313287
Test: atest FrameworksVpnTests
Flag: android.net.platform.flags.collect_vpn_metrics
Change-Id: I609d46dac77f7f5a3b760c69a3548b95ba8073ed