Add PRODUCT_SYSTEM_SERVER_JARS_EXTRA interface

This interface allows vendor to append their system server jars after AOSP's
without changing inherit order.

Test: booting
Test: build pass on branch r-fs-release with this patch
Bug: 140712566
Change-Id: I0ddf35bdf725c262f817985f5494b5968db21fcd
Merged-In: I0ddf35bdf725c262f817985f5494b5968db21fcd
(cherry picked from commit 2a703ce2)
Exempt-From-Owner-Approval: cherry-pick