[Storage management] Use Spaced service for total and free disk space

Use the new Spaced DBus method GetRootDeviceSize that returns the size
of the disk instead of the size of the current partition.

Use GetTotalDiskSpaceBlocking (statvfs) when the Spaced method above
fails (e.g. when the Fake client is used), and for guest profiles where
disk spaces are not relevant (in-memory temporary filesystem).

Stop rounding the total disk space to the next power of 2.

Refactoring: the observer StorageHandler::OnSizeCalculated is now
responsible for updating the map of items sizes
(storage_items_total_bytes_) and marking the correct item as calculated
(calculation_state_). It then calls dedicated functions to update
the UI (UpdateSizeStat, UpdateStorageItem, UpdateSystemSize).

Test: unit_tests --gtest_filter=StorageHandlerTest.*
Bug: b:173086361, chromium:1262859
Change-Id: I0ca5f6490bef44616ed5f743b07f010fb44c758d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3216578
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Jon Mann <jonmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#934374}