Add entries for STANDALONE_SYSTEMSERVER_JARS to systemserverclasspath.pb

This CL updates platform_systemserverclasspath,
systemserverclasspath_fragment, and
prebuilt_systemserverclasspath_fragment to write entries for
STANDALONE_SYSTEMSERVER_JARS to systemserverclasspath.pb. A new property
`standalone_contents` is aded to `systemserverclasspath_fragment` and
`prebuilt_systemserverclasspath_fragment` to list the standalone system
server jars in the APEX, and entries will be written to
systemserverclasspath.pb accordingly at build time.

To add more context, these entries will be consumed by derive_classpath
in order to generate an environment variable
PRODUCT_STANDALONE_SYSTEM_SERVER_JARS. The environment variable will
then be comsumed by odrefresh to determine what jars to preopt on early
boot.

Note that the variable should not end with "CLASSPATH" because the list
is not used by runtime as a classpath. It is just a colon-separated list
of jars. System server loads the jars separately with paths hardcoded in
the code.

Bug: 203198541
Test: manual -
  1. Add some jars to PRODUCT_STANDALONE_SYSTEM_SERVER_JARS
  2. Add some other jars to PRODUCT_APEX_STANDALONE_SYSTEM_SERVER_JARS
     and standalone_contents.
  3. Build an image.
  4. Flash the image to a device.
  5. adb shell echo \$STANDALONE_SYSTEMSERVER_JARS
  6. See the correct list of jars.
Change-Id: I09a6fd1d3db85c194330da9b751702a9bf069e26