GKI task entry points should have a consistent signature.

GKI task entry points should have a consistent signature.

Previously, GKI task entry points had inconsistent signatures. For
example, we had:

int btif_media_task(void *p)
void btif_task(UINT32 params)
void btu_task (UINT32 param)

The single argument was universally ignored and the caller always
set it to 0. This change consolidates all of that and defines the
entry point as having 0 arity and no return value.