Add constant value parsers.

To allow better readibility, we allow the property config file to
use string value to represent constant values. For example,
"VehicleProperty::INFO_VIN" represents the int VehicleProperty value
for INFO_VIN. This CL adds the parsing logic for special constant
format string values.

Test: See the unit tests added in the ag/19240183
Bug: 238685398
Change-Id: I1617fdd47b4292180bd2bb37acddc181c212f4f5