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Ignore null groupId when restoring default account.
When restoring accounts in sPhoneAccountXml.readFromXml, the groupId of the newly created accounts isn't filled (setGroupId isn't called on the builder) and thus none of the accounts have a group ID assigned. When then running setUserSelectedOutgoingPhoneAccount(), the used groupId consequently is empty, thus it doesn't seem correct to mandate it being non-empty when later restoring it. Change-Id: I14256616bae62e6689e0d2d4751f41bd90dfcba6