Dexpreopt standalone system server jars.

Standalone system server jars are dynamically loaded by system server
using a `PathClassLoader` whose parent is `SYSTEMSERVERCLASSPATH`. They
are listed in `PRODUCT_STANDALONE_SYSTEM_SERVER_JARS` and
`PRODUCT_APEX_STANDALONE_SYSTEM_SERVER_JARS` in Makefile. We need to
dexpreopt them to achieve better performance.

Bug: 203198541
Test: m nothing
Test: -
  1. Add a standalone system server jar (e.g., by patching
     aosp/1906158)
  2. Build a system image.
  3. See the odex and vdex files generated in
     $ANDROID_PRODUCT_OUT/system/framework/oat/
  4. Flash the image to a device.
  5. Run `atest art_standalone_dexpreopt_tests`.
Change-Id: I358a62d34989c5c8eba12e18fe6167e0b72ff69d