base_system.mk: Support system_server jars from apexes

Allow system_server jars delivered via apex. Regular system_server
jars are located in /system/framework folder. But, jars delivered via
apex are mounted at /apex/<module_name>/javalib. Also, not all the
libraries in /apex/<module_name>/javalib will be a system_server jar,
so adding a mechanism to list out the jar file explicitly within the
apex module.

Bug: 144722612
Bug: 141785760
Test: Compiles (both with empty & non-empty PRODUCT_SYSTEM_SERVER_APEX_JARS
value set)

Change-Id: Ia181ab22fdf2da575bfd532c1cd90a2f54742528