webapps: allow callers of icon downloader/selector to specify a minimum size
Since we now want to allow splash screen images down to 48dp but still want
to keep the one density bucket requirement below for home screen icons, the
callers should be able to specify the minimum icon size that they want
selected/downloaded.
Since it was going to be a ton of boilerplate to add this in the same way as
ideal_splash_icon_size_in_dp was added, also refactor the way icon sizes are
stored to reduce the boiler plate.
As a plus, AppBanner on desktop also gets minimum icon sizes correctly now
which was present as a TODO.
BUG=508627
Review URL: https://codereview.chromium.org/
1308533006
Cr-Commit-Position: refs/heads/master@{#349106}