Augment `flutter screenshot` with all supported screenshot types (#17478)

Previously, the only hook into the rasterizer schreenshot code path
was via FlutterDriver.screenshot.  This adds the ability for the
Flutter tool to hook into all three types of screenshots:

1. device (e.g. `adb shell screencap`)
2. Skia (capture spk)
3. Rasterizer (capture PNG via the engine)