Fix crash from constant data missing ID data

[jeffdgr8]

It's possible for StandardAddressVerifier.verifyFields() to crash with:

Fatal Exception: java.lang.NullPointerException: Cannot use null as key
       at com.google.i18n.addressinput.common.Util.checkNotNull(Util.java:123)
       at com.google.i18n.addressinput.common.FieldVerifier.isCountryKey(FieldVerifier.java:439)
       at com.google.i18n.addressinput.common.FieldVerifier.populate(FieldVerifier.java:171)
       at com.google.i18n.addressinput.common.FieldVerifier.<init>(FieldVerifier.java:105)
       at com.google.i18n.addressinput.common.FieldVerifier.refineVerifier(FieldVerifier.java:283)
       at com.google.i18n.addressinput.common.StandardAddressVerifier$Verifier.run(StandardAddressVerifier.java:132)
       at com.google.i18n.addressinput.common.StandardAddressVerifier.verifyFields(StandardAddressVerifier.java:86)

This happens if ClientData.fetchDataIfNotAvailable() fails to fetch data from the server in CacheData.fetchDynamicData(), resulting in getting the data from the RegionDataConstants with CacheData.getFromRegionDataConstants(). The constants data lacks the id key values that are returned from the server, which are required by FieldVerifier.isCountryKey().

This change fixes this by adding the id value for region constant data from its LookupKey.