Adding items to map.csv should not remove const fields.

[gugavaro]

Generator does not generate const fields for enum items. however if a current existent field gets added to the map.csv, generator will change the type and break api.
We need to add a flag on map.csv indicating whether field should be kept from being changed.

for example:

Original const field:

[Register ("EFFECT_CLICK", ApiSince = 29)]
public const int EffectClick = (int) 0;

if this line is added on map.csv

29,Android.OS.VibrationEffectEffectType,EffectClick,android/os/VibrationEffect.EFFECT_CLICK,0
then generator will generate the following field:

[Register ("EFFECT_CLICK", ApiSince = 29)]
public const Android.OS.VibrationEffectEffectType EffectClick = (Android.OS.VibrationEffectEffectType) 0;