Create template splash screens for Android and iOS

[xster]

Fixes #8147

This add 81kB to a newly flutter created project.

[Hixie]

iirc @abarth had opinions about doing this

[xster]

I'll add some splash screen instructions (probably to https://flutter.io/assets-and-images) in a separate website PR

[Hixie]

I would rather we didn't show the Flutter logo by default. Our developers' apps' users don't care that our developers use Flutter. (Also if we do decide to show the logo, we should show the right one -- these PNGs have the SVG rendering error.)

Can we make these default splash screens just black, maybe?

[xster]

You're right, the svgs do look broken. Seems like the illustrator vectors in the same directory are ok. Swapped out assets for those one.

It's not so much about showing the flutter logo as having a template for developers to drop in assets for splash screens without having to do all the work that's in this PR.

We can document on the website how to remove image assets to just have static colors but I think for developers, removing will be a lot easier than adding. But I think we can use a different image placeholder if there's another one you prefer.

[Hixie]

I don't mind having a placeholder image, I just think it should be all-black or something like that.

Many developers are just writing little apps for their own personal use. They don't want to have to go and remove our big logo to have a professional-looking app. They just want an out-of-the-box polished experience. It's bad enough that we use our logo for the icon. ;-)

[xster]

Did you mean a black box in a white background?

[xster]

Made the changes we talked about, removed the actual assets. Left the hookups.

[Hixie]

The approach LGTM. I'm totally ignorant about the actual files you're modifying. :-)

[xster]

I tweaked it again. The iOS side was giving warnings in Xcode and sometimes while running. Felt unclean. Added back a 1px transparent png for placeholder.

[coveralls]

Coverage decreased (-46.7%) to 38.06% when pulling 46038d9 on xster:splash_screen_template into 4cb3a98 on flutter:master.