SplashScreenAnimated support

[FabienZE]

Please support the full new Android 12 Splashscreen API
with these 2 attributes :

<item name="windowSplashScreenAnimatedIcon">@mipmap/ic_launcher_round</item>
<item name="windowSplashScreenAnimationDuration">300</item>

as it is documented on Google Docs :
https://developer.android.com/develop/ui/views/launch/splash-screen#implement

[shatanikmahanty]

@jonbhanson may I work on this? I have a working code using the example starter kit provided in official android sample. Would try to integrate that then!

Changes estimated to be required by user:

• Need to set min android sdk constraint 21 for the android dependency required to work properly. User also needs to use the above mentioned library in android build.gradle

• Need to set two parameters in pubspec.yaml config corresponding to the required parameters mentioned by @FabienZE for the library to add values in style.xml

Design doubts:

• Should the AVD file be used directly from android drawable folder or from assets folder in project root where pubspec exists?
• Should we use a default duration in case user doesnot specify the duration or we should show error in that case?
• Expected behaviour for Android 12 and above: When both AVD and image are provided. Should we show warning that android will directly use animated icon and ignore the splash icon vector, or we should let it be?

[jonbhanson]

@shatanikmahanty anybody can add features. As long as some in the community want the feature and the code quality is high, the PR will be accepted.