Add RO-RTC support to system-image

Created by: UniversalSuperBox

Most newer Android devices have a read-only hardware clock. This means that we need to store an offset value from that clock to the current time and reload it at system startup to retain a correct time. Unfortunately, our recovery doesn't do that. Instead, it just keeps the default time (which is usually no later than sometime in 1975). This causes a small artifact when querying for system-image's last updated metric, the date is shown as sometime in the 1970s rather than when you actually updated.

This commit makes system-image read the offset from timekeeper (ubports/timekeeper) and use that when it determines the time of the last update. Fixes ubports/ubuntu-touch#685 (closed)