Make car apex's library optional in PRODUCT_APEX_SYSTEM_SERVER_JARS

- car-frameworks-service-module is added from com.android.car.framework
  apex which only exisits in car products.

Bug: 203233647
Bug: 203201931
Test: build, bootup
Change-Id: I5f13b0505c1ac4fd2fd7b6559e080c8a6fb526e0