init: Use android::base::boot_clock instead of /proc/uptime

/proc/uptime internally uses whatever would be returned by
clock_gettime(CLOCK_BOOTTIME), so use android::base::boot_clock instead
which avoids parsing strings and rounding errors.

Bug: 77273909
Test: CtsBootStatsTestCases
Change-Id: Ic162eefcf226073949a18cca55db3c2324b98749