Make bootstat container-friendly

This change allows bootstat to read the ro.boot.boottime_offset
property, which is set on devices where Android runs in a container.
This is because the CLOCK_BOOTTIME clock is not reset when (from
Android's perspective) the device restarts, since the host OS does not
restart itself.

Bug: 77273909
Test: CtsBootStatsTestCases

Change-Id: Ifb792864394be0b4686cc9d555c0afced856f4b4