Minor code cleanups around filterValues() for field filtering

- Instead of output parameter, vector<FieldValue>*, return the result as
  value to take advantage of Return Value Optimization.
- Move fieldMatchers.empty() check inside filterValues to avoid
  duplicating the check at the callsites.
- Get rid of unnecessary shared_ptr for GaugeAtom::mFields. Ownership is
  not shared.

Bug: 376872102
Test: statsd_test
Change-Id: Ie571bfa45c7477f81839d6120ed66c968fec0877
7 files changed