Alert thresholds should be coherent in metric <=> imperial

[lzpons]

for any kind of alert, when a user changes the alert threshold in a unit system, the change has to be translated also in the other unit system.
Currently (bug) change is not translated to the other unit system and if user changes unit system it will recover the default alert thresholds, whatever the user has changed on the other system.

Required change:
improve java backend to translate threshold changes to the other unit system too. in practice both fields valueMet and valueImp should be coherent with maybe some extra rounding to avoid excessive decimals.