Support for US Privacy flag

[yoalex5]

1. CCPA was added Support for US Privacy flag #250
   API:

//add
UserDefaults.standard.set("1YN", forKey: "IABUSPrivacy_String")

//remove
UserDefaults.standard.removeObject(forKey: "IABUSPrivacy_String")

OpenRTB request:

"regs": {
  "ext": {
   "us_privacy": "1YN"
  }
}

2. fixed GDPR exception support for gdpr consent #243
3. subjectToGDPR can be assign to nil to get an IAB value

Targeting.shared.subjectToGDPR = nil

4. refactoring
5. tests were added

[anwzhang]

Should we call this PrivacyUtils?

[yoalex5]

StorageUtils is a general utility class to work with persistent storage. It will allow us to expand this class in future without generating a lot of small classes. If we had several big logic entities I would create PrivacyUtils, StorageUtils etc.

Please keep it as is and we will change it when it will be necessary

[ppuviarasu]

Is this object ext["us_privacy"] created even if the ccpa is empty or nil?

[yoalex5]

If StorageUtils.iabCcpa() returns nil or empty string the ext["us_privacy"] will not be created

Check tests: testPostDataWithEmptyCCPA and testPostDataWithoutCCPA