Simplify time handling internally and for build scans (#2857)* Don't make TimeProvider Serializable.This isn't safe and generally doesn't make sense.* Extract and promote the concept of a build timer.This was previously not well defined and being overlaid with the concept of when a user/tool requested something, which is not always the same thing.* Pare down the deprecated org.gradle.util.Clock down to the minimum required.Internal usage is replaced by a `getStartTime()` directly on BuildRequestContext.What is left is only kept for backwards compatibility with scans.* Rename TimeProvider to Clock.* Move BuildExecutionTimer out of baseServices into core, and into a better package.* Remove unused.* Simplify the time package by merging types.* Prevent the client's build started timestamp from being later than when the provider received the build request.* Provide a dedicated mechanism for conveying the build start time to build scans.* Consolidate the ways of formatting durations.