Well, sometimes it helps to just approach the problem again after you explained it to others… Here is the solution I got the problem solved with:
# make a copy of the original activation_map
nact = act.copy()

#Reshape the activation map to a 1D Matrix / Array
nact = nact.reshape(-1)

#Get the indices that would sort the array from low to high
sort_ind = np.argsort(nact)

# indices of the values I want to set to 0
sort_zero = sort_ind[:47175]

#indices of the values I want to set to 1
sort_ones = sort_ind[47176:]

#set values to zero
nact[sort_zero] = 0.0
# set values to one
nact[sort_ones] = 1.0

#Reshape activation_map back to original size
d_act = nact.reshape(224, 224)

#Visualize it over the image
plt.imshow(img)
plt.imshow(d_act, alpha=1.0, cmap=my_cmap)
plt.axis('off'); plt.tight_layout(); plt.show()

Result: