e-Chords displays their primary interpretation of the song Angie as it being in the key of Am.
http://www.e-chords.com/chords/the-rolling-stones/angie
My bandmates and I (none of us properly school-trained, or able to read a note of sheet music) generally call a song 'in the key of' whatever cord the song starts out in.
For instance, we could play JJ Cale's Call Me The Breeze in either the key of A or the key of E, depending upon which one of us in the band felt like singing the lead.
If we kicked into that song then, one of us would call out, "Call Me The Breeze in A," and then we would play in the three chord progression of A, D, and E.
If our girl singer says, "Call Me The Breeze in E," we would proceed with the performance in E, A, and B.
Key of C, in that same blues progression, would be C, F, and G.
It may not be proper, but that's how we do it.
And trust me, whatever the girl singer wants, she gets.
She's a regular diva, that one.
:(