How I solved it - Open an image editing software (Photoshop) and erase the things that you don't want in the icon ( this means the background , logos and other unwanted things). The reason is, small icon of android notifications in the mobile notification settings of Unity can only be shown by using alpha channels. further more, the parts of the image or texture which aren't transparent will be seen and the parts which are transparent wont be seen. So, you might understand that this will be a black&white icon. When you are editing in the image editing software, leave everything that you need to be scene, their color doesn't matter cuz at the end they will be just white.

If you want a background, try changing the alpha values of the background and the icon( the particular shape which is considered as the icon , ex - A sword as the icon and a background for it)
if you make the background's alpha values less, it will become a bit transparent than the icon(sword in my example), if you do the opposite, let's say you deceased the icon's alpha value, and increased the background, the opposite would happen. [ I didn't practically check this but according to my knowledge this should work)