Only allow clearing NAT timeout when the 25Q2 flag is enabled

This commit flags off the API behavior change that was meant to
be added only to 25Q2, but was introduced to older SDKs and caused
released CTS to fail.

Bug: 379947714
Test: atest FrameworksVcnTests && atest CtsVcnTestCases
Test: verified against Android15 CTS
Flag: android.net.vcn.mainline_vcn_module_api
Change-Id: I39d33fa80225b138d64f18819eff07f5ef156dd1