btaa: update hardware include file to be gd ready

Move Activity enum inside ActivityAttributionCallbacks class as this
include file will not cross boundary and be available for the btaa
module within the gd. Rename the Init() function, as btaa within gd
will be modularized, hence the stack will be responsible for the
initialization.

Tag: #feature
Bug: 172501038
Test: m
Change-Id: I371de5399abbfb0929baff0695ea1608e2341347