Support SYSTEM_SERVER_JARS_EXTRA in dexpreopt

Updates hardcoded /system path to use
the qualifier before colon to support
/system_ext paths in dex_preopt.mk.

Bug: 155630745
Test: m dist droid with a system server
  library on a system_ext partition

Change-Id: I4dccc1f55bd2cafa13d57b46f122c44f66da335b