Unify permissions under UPDATE_DEVICE_STATS.

The UPDATE_DEVICE_STATS permission has become the de-facto mechanism
that platform components use to shift blame for resource usage, so
it's confusing to also have a separate MODIFY_NETWORK_ACCOUNTING
permission.  So this change replaces MODIFY_NETWORK_ACCOUNTING with
UPDATE_DEVICE_STATS.

Bug: 62483389
Test: builds, boots
Exempt-From-Owner-Approval: Bug 63673347
Change-Id: I872759f02327b6d531ec2338bd876890aded60ad