Remove DeviceConfig usage from NetworkStackClient

DeviceConfig API is not yet submitted. Use Settings.Global instead, to
still allow writing tests against AOSP code (a test would try to use
adb shell device_config, and fallback to adb shell settings).

This is not merged anywhere else, the merged-in is here to ensure this
does not end up in branches that use DeviceConfig. This change should be
lost when AOSP is updated.

Test: flashed, force-crashed NetworkStack with different setting values:
      observe rate-limited crash
Bug: 133725814
Merged-In: I423ca6ebb328f49b170baae0da9b8409a6429fcb
Change-Id: I399d3e37f1faaecb8a30428c1989fac8821379d8