### Description of the changes:
**1) Do not set units to default values if they already have valid values**
This fixes the actual issue. `UnitConverter::InitializeSelectedUnits()` ( this function resets all units to their default units if available for the current category ) gets called after `UnitConverterViewModel::RestoreUserPreferences()` ( this function restores user preferences ).
So Calculator has been restoring saved values, and then overriding the restored values with default values.
I modified `InitializeSelectedUnits()` so that we only initialize units only when they are not already set to valid units for the current category.
**2) Removed `m_isFirstTime`**
I noticed that we are calling `RestoreUserPreferences()` twice when Calculator starts up, and the function is restoring the same value both times
The below happens when Calculator starts up
1) On startup, in `UnitConverterViewModel::InitializeView()`, `RestoreUserPreferences()` is called.
2) `RestoreUserPreferences()` in turn triggers `OnUnitChanged()`
3) During the first call to `OnUnitChanged()`, m_IsFirstTime is `True`, so we call `RestoreUserPreferences()` again while also setting `m_IsFirstTime` to `False`.
4) `RestoreUserPreference()` again triggers `OnUnitChanged()`
5) During the second call to `OnUnitChanged()`, m_IsFirstTime is `False`, so we call `SaveUserPreferences()`
I think we should only call `SaveUserPreferences()` inside `OnUnitChanged()` since we already restored user preferences during view initialization. I can't really think of a reason to restore units after view has been initialized. This led me to just delete `m_isFirstTime`.
### How changes were validated:
Manually tested that units and the current category are properly selected when you quit and start Calculator.
## Fixes#445.
af8322617f
