Always allow root uid to update device stats

root uid should always have android.permission.UPDATE_DEVICE_STATS, so
we skip the permission check if the calling uid is 0. The request from
root uid will not get blocked even if the framework doesn't pass down
the correct permission.

Bug: 111560570
Test: netd_integration_test
Change-Id: Idb535f02faadc3e0cb3c14b18e82742aa8976fac