[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.
`os.path.isdir`