Instantiate the DWA service.

* Instantiate the DWA service if kDwaFeature is enabled.
* Checks for sync/UKM consent and incognito before enabling DWA recording.
* Purges entries if consent goes from enabled to disabled, and/or incognito is activated.
* Excludes iOS.

Bug: 376265368
Change-Id: Ia39ec4a6f206486fede0c8baa57c79d5edf6de45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6027152
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Zainab Rizvi <rizvis@google.com>
Commit-Queue: Eli Hayoon <elius@google.com>
Cr-Commit-Position: refs/heads/main@{#1398873}