Don't call into system_server for permissions check

With the recently added permission information in traffic controller,
netd can check if the calling process has permission UPDATE_DEVICE_STATS
without calling into system_server. Update the code path and add some
test cases for it.

Bug: 111560570
Bug: 111560739
Test: netd_unit_test, netd_integration_test
Change-Id: I79eee1321f32154e91466f023f7952db23df8494