[NewEV] Add getFileSystemInfo private Extension API

Added the new private extension API to be used by the SecureConnect
extension for getting device signals. The API will do the signal
collection through the SignalsAggregator keyed service.

Bug: b:237526563
Change-Id: Ie43c3ad58230eda42cd8bb1d48b49290a87a1651
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3750906
Reviewed-by: Hamda Mare <hmare@google.com>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1023932}