Add support for controlling the "compat" app icon shape

[mattleibow]

Description

In Android 8.0 (API 26), the OS added support for adaptive icons. Resizetizer handles this just fine, however when it generates the backwards compat icon, it is just a square - or more specifically it is just drawing the background as is.

The Android 8.0 OS has some options:

But for older version of the OS, the developer is responsible for generating the "shape". For example, in Android Studio:

As you can see, there is both a round icon and a legacy icon. We already have the round adaptive icon, but still need to add the legacy versions of both the actual round image and then the shaped image (square - current, squircle, rounded square, circle)

There is also an app that can be used to test many variations: https://github.com/nickbutcher/AdaptiveIconPlayground

In summary, we have:

• [x] Adaptive shaped
• [x] Adaptive round
• [x] Legacy none

We are missing:

• [ ] Legacy round
• [ ] Legacy shaped (square, squircle, circle, rounded square)
• [ ] Legacy alternate shaped (vertical, horizontal)

Public API Changes

Maybe all we need is an optional parameter on <MauiIcon Shape="xxx" />

This would then be a choice of a few things:

• None (current and default)
• Square
• Circle
• Rounded Square
• Squircle
• Vertical
• Horizontal

Intended Use-Case

More things to support typical scenarios.