Adaptive icon support

[lokesh-krishna]

This adds the new icon from #638 but now with support for adaptive icons on devices running Android 8.0 or higher. The new icon in the shape of a rounded square is used as the legacy icon to be shown on devices running older versions of Android.

The icons have been moved from the drawables directories to mipmap directories as recommended here.

This is what the new icon looks like on legacy devices:

The adaptive icon looks different depending upon the mask used by the launcher or selected by the user:

As discussed earlier, we'll need to add credits for Michael Cook in About.
I hope it is okay to have updated the readme icon as well in this same PR.

We will need to build against SDK 26 in order to support adaptive icons though we don't need to target API 26.

[Nuntius0]

The build fails because it can't find @mipmap/ic_launcher as specified in the manifest, because it only looks in main/res for mipmaps. If you move the mipmap folder there, it'll works.

As far as I understand it, the different resource folders are only ever used to provide a different icon for the debug flavour. What about unifying all those, and specifying the right app icon to be used in gradle?
That would a) fix this issue, and would b) clean up the folder structure.
https://stackoverflow.com/questions/22875948/how-to-provide-different-android-app-icons-for-different-gradle-buildtypes

[lokesh-krishna]

I would like to hear what the devs think about this. Should I move the mipmap folder or would it be a better solution to clean up the folder structure?

[Kdecherf]

@Strubbl any thoughts?

[Strubbl]

I like it very much, that you give credits to Michael in the Readme.

I like the approach @Nuntius0 is describing. A cleanup of the folder struture w.r.t. the icons sounds good to me.
Let's go for that. 👍