Add a runtime check to ensure that system server jars are prefetched.

We prefetch standalone system server jars in ZygoteInit based on the
STANDALONE_SYSTEMSERVER_JARS environment variable, so that they can take
the advantage of AOT compilation. This CL adds a check to disallow jars
that are not prefetched, which reminds developers to make appropriate
changes so that their jars will be in the environment variable.

Bug: 203198541
Test: 1. Build a system image.
  2. The device boots.
Test: 1. Remove an entry from PRODUCT_APEX_STANDALONE_SYSTEM_SERVER_JARS
  2. Build a system image.
  3. The device does not boot and encounters the following error:
     java.lang.RuntimeException: Creating a ClassLoader from /apex/com.android.wifi/javalib/service-wifi.jar is not allowed. Please make sure that the jar is listed in `PRODUCT_APEX_STANDALONE_SYSTEM_SERVER_JARS` in the Makefile and added as a `standalone_contents` of a `systemserverclasspath_fragment` in `Android.bp`.
Change-Id: I275d75ac37194a4d8fd491529b7cdb697dc04e37