Report unavailable data usage.

- when showing the data usage graph, if the first data point is equal to
the first available data, insert a 0 usage point on the first day of
available data, so that it will show a flat line for the days that do
not have available data instead of showing a non-zero slope.

Change-Id: Ibf305e292a65ab3310ea12efbb575e3a7e1b6012
Fixes: 76155147
Test: make RunSettingsRoboTests