Always prepend idfa to targeting identifiers

[eugenedorfman-optable]

Objectives

1. Always add idfa as the very first identifier in the list of identifiers we send in the targeting array (prepend the list).
2. If it is also added by the user as part of the identifiers list - remove it to deduplicate - only send the system one prepended to the list.
3. Add a test verifying that it is prepended when idfa is available (tracking authorized) and not disabled by the user through config.
4. Add a test verifying that it is not pretended either when it is disabled in the config or when unavailable due to ATT.

[vladislav-yermakov]

@eugenedorfman-optable,
1 What should sdk do if user provides a list of idfa-s? (merge? – if yes, what the merge logic should be)
2 What if system idfa is not available, but user provides 1-N idfa-s? (prepend all?)
3 What if system idfa and user provided idfa does not match, does sdk should prepend like this; [SystemIDFA, UserProvidedIDFA, ... ]?
4 What if skipAdvertisingIdDetection is true (automatic IDFA detection is skipped), but user still passes the IDFA, should sdk prepend it to other ids?

[eugenedorfman-optable]

Good questions @vladislav-yermakov. I'd like @zapo's input on these behaviors, my take is this:

1. If user provides a list of IDs - we don't alter it, it will be sent as user has provided it, all we do we just prepend the retrieved IDFA if it was retrieved (i.e. no Limit Ad Tracking).
2. The above answer covers this case as well I believe
3. The above answer covers this case as well I believe
4. Same as above, no alteration to the list of user-supplied idfas, just send them as is, only prepend when system one was available and retrieved.
   cc: @jrosendahl-opt