[MS66] Initialize TrafficStats with context

TrafficStats has static methods created in API level 8 that need
access to NetworkStatsManager but doesn't take a context.
Previously this was achieved by using ServiceManager, but
with TrafficStats moving to the connectivity module, this
is no longer possible.

Instead, make sure TrafficStats has an appropriate context by
the time any client code can call the relevant methods.
• In app code, this achieved by passing the application
  context from ActivityThread#handleBindApplication, before any
  app code can run.
• In the system server, this is achieved by passing the context
  right after creating service.

Test: atest TrafficStatsTest CtsWebkitTestCases
Bug: 204830222

Change-Id: I251bb8a4431ad12ff61929879ef1363cf06b9244