shill: cellular: Enable Verizon OTA LTE activation

1. Removed dummy service used for activation.
2. Immediately reset the modem after activation, removed code that
waited for registration before resetting the modem.
3. Removed activation state assumptions based on registration/connected
state.

BUG=chromium:384185
TEST=Unit tests, manually activate Verizon LTE on Link and Spring

Change-Id: Ibd7c402f97618c70dd0fad0ce1e75e2eb4284aa6
Reviewed-on: https://chromium-review.googlesource.com/207680
Reviewed-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
8 files changed