soong: 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.

Bug: 144722612
Bug: 141785760
Test: Compiles
Change-Id: Ia40bb91d2e05b2601a52eac28a985fe2d8da3481