Check clock accuracy before attempting to display clock

The RTC may not be set yet, may have been reset, or may have drifted
significantly if the device time hasn't been updated in a long time.

Using the the last_time_change file to determine if the time is known to
be accurate and only display the clock when that is the case.

Bug: 27802041
Change-Id: I845cf9c74fcb4009504f1bab853e04146fa19e1d