Move UkmObserver out of UkmDataManagerImpl

UkmDataManagerImpl is created when kSegmentationPlatformUkmEngine
feature flag is enabled. However, UkmObserver is needed for histogram
based ML models that may not have the feature flag enabled.

This CL:
1. moves UkmObserver into UkmDatabaseClient, so it can live
independently from the UkmDataManagerImpl.
2. Removes UkmDataManager::NotifyCanObserveUkm(). This method is not
needed since metrics service is created before UkmDatabaseClient so
the observer can created before UkmDataManagerImpl.
This also simplifies the logic in UkmDataManagerImpl to not cache
pending configs.
3. Removed UkmDataManager::StopObservingUkm(), putting the method
 into UkmObserver since now UkmDatabaseClient manages it.


BUG=1298641

Change-Id: I324e0d39911ff2280b06db87f3bc3a470d084f61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3585073
Reviewed-by: Siddhartha S <ssid@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#993010}