gradle: set the targetSdk/compileSdk/buildToolsVersion/minSdk globally

So we don't have to set it in every single sub folder.

We have to keep the values in rootProject.ext for libvlcjni.

And use minSdk instead of minSdkVersion to make sure it's an integer value.

The minSdk may get the same treatment, I need to check what happens with vlcBundle when we override the value to 30 (from 17/21).