Add "contents" property to systemserverclasspath_fragment.

Similar to bcp_fragment's contents, this property lists all java library
contributions made by this fragment.

Bug: 180105615
Test: m nothing
Merged-In: Ifb1f54d5db290fffaa31933d15207014bb72d2fb
Change-Id: Ifb1f54d5db290fffaa31933d15207014bb72d2fb
(cherry picked from commit 9366a053)