As I understand it, the majority of Afro-Americans are from families who have been in America for a significant amount of time, as the result of the slavery practices of past centuries. The majority of slaves (again, as I recall the history) came from the West Coast of Africa, on the shipping route from Europe - not the East Coast where Swahili is spoken (Imani is Swahili).
Additionally, the name only reached the top 1000 in the US in 1995 for boys and 1997 for girls. If it had been a traditional name in the Afro-American families it could be expected that it would have had a longer-term useage.
It is also significant that the name emerged first for boys, which makes no sense if it is being used in its feminine Imani derivation.
It seems almost certain to me that the use of Armani in the US is derived from the brand name, not from the Swahili feminine name.