Added image path for icons when toggle to dark theme

[Darshan-upadhyay1110]

Change-Id: I14a99df7cf10bf73f898f15d3aaa5a32c9cd51af

• Resolves: branding icon folder #4736
• Target version: master

Current case when we toggle to dark mode icons are not getting change because we are using default image path when we call get image url.

so i changed it according to the theme we are using and if icon does not present in dark theme then use the default icon.

@pedropintosilva , @eszkadev => need your review for this changes i made

[eszkadev]

looks good :)

can we do one more improvement:
merge L.LOUtil.getImageURL and L.LOUtil.checkIfImageExists into one function ?
something similar to:

L.LOUtil.setImage(img, name, doctype) {
    img.src = L.LOUtil.getImageURL(name, doctype);
    L.LOUtil.checkIfImageExists(img)
}

then we have to use one function instead 2 :)

[eszkadev]

works as expected :)

[eszkadev]

looks good, @pedropintosilva if you are ok we can merge

[pedropintosilva]

Thanks, pulling now to test

[pedropintosilva]

Thanks!