[SP17] Wait for stats providers to report stats update

From current design, the traffic accounted by stats providers
will be updated asynchronously when force polling. When upper
layer make two subsequently queries. They will get stale
stats upon the first query, and may get newest/stale stats
base on the result of race.

Thus, wait for a bit of time to allow asynchronous stats update
complete to reduce the chance of race. In pratice, it would
be finished in ~2ms when testing.

Test: systrace.py network
Test: atest FrameworksNetTests
Bug: 147460444
Change-Id: I22a00fc4049cddf77fd578e25769ae1979f2cc6d