DoH: Factor out BootTime

* Move `BootTime` into the `boot_time` module
* Change `elapsed()` to match `Instant` API
* Add timeout + sleep functions, relative to `CLOCK_BOOTTIME`
* Change everywhere in DoH to use `boot_time` instead of `time`
* Add tests for boot_time module

BYPASS_INCLUSIVE_LANGUAGE_REASON="man is referring to the unix manual command, not a person"
Bug: 202081046
Bug: 200694560

Change-Id: I719965ff75abb0223ba20829ca0a3a4be1d07f40