[Android] make_apk: Make the code handling icons easier to read.

`customize.py` is horrible to read, and `CustomizeIconByDict()` is one
of the biggest offenders.

The first two commits does some clean-up that allows us to get rid of
some checks or moves them to an earlier stage in make_apk.

The last commit rewrites `CustomizeIconByDict()` to make it easier to
follow, mostly by getting rid of `drawable_dict` and all the attempts to
be smart by over-manipulating it.
> Hmm, this is slightly tricky and I hadn't thought of that. Is that why that weird sorted() call was there? The Chromium manifest spec isn't clear about whether this is done there as well, so I'm not sure it makes sense to do this or not.

The problem is that there is no definition how to map size to size_name, we are just doing this for some common sense here. If two quite sizes are quite close, they may fit into the same size name. But for android, it only accept one image for each size name. Current rule is the later one declared in manifest win.

Anyway, it's kind of corner case and not that important.