bootstat: Refactor init/utils/boot_clock into base/chrono_utils.

Use this for bootstat and init. This replaces the custom uptime parser in
bootstat.

This is a reland of aosp/332854 with a fix for Darwin.

Bug: 34352037
Test: chrono_utils_test
Change-Id: Ib2567d8df0e460ab59753ac1c053dd7f9f1008a7