Conditionally add the car bcpf to platform bootclasspath

Previously, the com.android.car.framework was always added to the
platform bootclasspath and then Soong ignored errors that arose when
the android.car-module was not listed in PRODUCT_APEX_BOOT_JARS. This
changes uses the target_board_auto Soong config variable to control
whether the car fragment is added or not.

Test: none
Bug: 264598408
Change-Id: Ie2819215ae399cf50ff17ca47551947773cae80d