Refactor `NetworkConditionService` as nonsingleton

Refactor `NetworkConditionService` as non-singleton. Instead, an
instance should be created either as a member of an `UploadClient` or
`EncryptedReportingServiceProvider` object (in a future CL). This should
be able to ease future extensibility and testing.

Bug: b:214039157
Change-Id: Id8820c8bad38dc0089d867a9aa9b643b983f5d6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3600866


Reviewed-by: Leonid Baraz <lbaraz@chromium.org>
Commit-Queue: Hong Xu <xuhong@google.com>
Cr-Commit-Position: refs/heads/main@{#995403}