This is not safe to do in system server because it affects all
system server callers of NetworkStatsManager, including ones
that frequently call into NetworkStatsService, such as
NetworkPolicyManagerService.
Also, setPollOnOpen is unnecessary because data usage is only collected when processing the `onLost` callback, which is triggered after ConnectivityService forces a poll.
Fix: 427909947
Flag: EXEMPT BUGFIX
Test: covered by existing tests
Change-Id: I2fb25f4529d46dfc14b00b0aa69c5145317ca9dd