Fix updating and deleting FDN entries with an empty alpha identifier.

- Fix AdnRecord.buildAdnString() to generate the correct record when alpha
identifier is empty. This allows the user to update an FDN entry to remove
the alpha identifier. Previously the entire entry would be deleted because
an empty record was generated here when the alpha identifier was empty,
rather than a record containing the phone number with an empty alpha tag.
Also, return null if the number or alpha tag are too long.

- Fix bug in IccProvider.delete() where efType was compared against local
FDN constant rather than IccConstants.EF_FDN. This would always return
false. Comparing with IccConstants.EF_FDN gives the intended behavior.

Change-Id: I0ea75d7e107c7318c9a48ae6e0a15845a718f4c0