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/338325 with a stubbed implementation for Darwin.

This change also has clang_format fixes (automatic).

Bug: 34352037
Test: chrono_utils_test
Change-Id: I72a62a3ca1ccfc0a4ccc6294ff1776c263144686