Reland: Add sync device info observer for segmentation. Record uma metrics of device count by os types.

Update segmentation service factory for iOS.

- Derives OS types from DeviceInfo data from Sync.
- Adds a segmentation observer to device info tracker.
- Counts devices by OS types.
- Records UMA metrics for device count by OS types.
- Adds a Finch flag for active days threshold.

Previously reverted change: https://chromium-review.googlesource.com/c/chromium/src/+/3965194

Bug: b/254338267
Bug: 1381215
Change-Id: Ie097ec391e26ca518e36a08af2df254b1c55c30a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4004226
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Jun Zou <junzou@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1068410}