WifiEnterpriseConfig: Refactor set/get field values

WifiEnterpriseConfig has a few keys which are generated internally
have unquoted values. However, the public setFieldValue() always quotes
the value when set.  So, this causes a problem when restoring
these field values from config store. Since this is an internal
disctinction that only WifiEnterpriseConfig understands, add a list to
check if a particular field value needs to be quoted or not. Also,
remove any direct "mFields.put" invocations with |setFieldValue|.

While there,
Refactor the existing |setFieldValue| & |getFieldValue| methods.

BUG: 31246524
TEST: Unit tests
TEST: Integrated with config store and verified that a previous TLS EAP
connection failure is no longer seen.

Change-Id: Ib85f3bce378720a6a6c2ae1439837a8e866a088d