unable to make custom splash screen like youtube for android

[fullflash]

because of adobe preventing changing android:theme it is currently impossible without hacking apk

and making app specific preview window as described here https://cyrilmottier.com/2013/01/23/android-app-launching-made-gorgeous/

adobe can make a custom solution if like to prevent specific tags.

instead of default blank preview screen.

[Flipline]

This isn't quite the same thing you're talking about, but there are some tutorials for modifying the AIR SDK to get some sort of splash image like this one from distriqt:

AIR Native Extension Tutorials by airnativeextensions

Unfortunately you have to make these changes each time you download a new AIR SDK version, and all of the apps you publish will use this same style of splash since there's no way to do this on an app-by-app basis in the manifest.