Fix build file such that CarSystemUi can be easily extended

This is done buy building all of CarSystemUI as a library and then including that
library as a static lib for an android_app target. Thus anyone that wants to
build a derivative to CarSystemUI can include that library and start extending.
It is the same mechanism that allows CarSystemUI to extend the phone SystemUI

Bug:140875212

Test: Build and Deploy
Change-Id: I4c83b0147c5ce8a8b4321ba0866721e4922df431