[FET] Field trial testing config for enabling OnDeviceStorage

This CL enables the feature OnDeviceStorage in testing. The feature adds
a new device storage for feature engagement events.

Change-Id: Iacc3b457c400c9ae6b3eee843489484c6eb96b13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6656669
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Cheick Cisse <cheickcisse@google.com>
Cr-Commit-Position: refs/heads/main@{#1477494}