Add server flag for the lower bound

Add a server flag for the lower bound time used by the time_detector to
validate times in suggestions. This can be used during tests to confirm
that suggestions are properly compared against the lower bound before
being used, and to bypass the lower bound check when running tests with
historical data.

Note: Currently, the lower bound is used to validate / filter
suggestions when they are received. Invalid suggestions are discarded,
leaving old suggestions. Changing the lower bound will not cause the
time_detector to discard / ignore already accepted suggestions if they
now fall below the lower bound.

Testing:

adb shell dumpsys time_detector # Look at reported lower bound
adb shell device_config put system_time \
    time_detector_lower_bound_millis_override 0
adb shell dumpsys time_detector # Look at reported lower bound
adb shell device_config delete system_time \
    time_detector_lower_bound_millis_override
adb shell dumpsys time_detector # Look at reported lower bound

Bug: 172229867
Test: See above
Change-Id: If1d5fd1aa09732c05808cc13dd257f2daeea6220