altoslib,altosui: Be more robust when graphing bogus .telem files

Deal with files containing multiple serial number/flight number values
by preserving the boost_tick value across state resets.

Check for invalid state when computing actual boost time for the stats
window.

Ignore invalid speed/accel values when computing averages.

Signed-off-by: Keith Packard <keithp@keithp.com>