CSD: fix issue with timestamp after reboot

The CSD record timestamps use CLOCK_MONOTONIC which is counting from
boot time. In order to have a consistent timestamp between reboots we
need to convert between global system time to boot time.

Test: audio and media.audio_flinger dumpsys after reboot
Bug: 266915998
Change-Id: I49111f002d95c27c039e1bd719c39ab5ae5e29d3