Add a variable to specify standalone system server jars.

This is the list of jars that system_server loads dynamically using
separate classloaders. We will rely on this variable to decide which
jars to preopt in the build system and on the device for system_server.

The list is supposed to be in sync with the code in SystemServer.java.
There will be a follow-up CL to add a comment in SystemServer.java to
remind developers to keep them in sync.

Bug: 203198541
Test: m nothing
Change-Id: I305a73218ef2d2c61ac3795d21026b2afe7007fd