prefetch: Derive default path/ready_path from tracing_instance

Previously, the default paths for the `.pack` and `_ready` files were
hardcoded. This required every prefetch service definition in the `.rc`
files to explicitly provide a unique `--path` argument to avoid
different services overwriting each other's data.

This change makes the default path dynamic by deriving it from the
`--tracing-instance` argument when `--path` is not provided. For
example, a service with `--tracing-instance prefetch_apex` will now
default to using `/metadata/prefetch/prefetch_apex.pack`.

To implement this, the `--path` and `--ready-path` arguments are now
optional, and the logic for resolving the final path is encapsulated
in new `get_pack_path()` and `get_ready_path()` methods on the
RecordArgs struct. The rest of the codebase has been updated to use
these new methods.

Test: Manually verified that starting different prefetch services
creates distinct .pack files based on their tracing instance.
Bug: 436145792

Change-Id: I5e9afa4042e293b31b6cb8355946ee9e402c5a38