Add systemserverclasspath_fragments property to apex.

- All contents of the fragment are added as java_lib dependencies.
- Generated classpaths.proto is added into etc as required.

Bug: 180105615
Test: m nothing
Merged-In: I8e8e8b019c4ca2909182f205a47deffa946de6da
Change-Id: I8e8e8b019c4ca2909182f205a47deffa946de6da
(cherry picked from commit 333a1732)