Dexpreopt standalone system server jars from prebuilts.

This change adds support for dexpreopting standalone system server
jars from prebuilts.

Bug: 203198541
Test: -
  1. Add a standalone system server jar (e.g., by patching
     aosp/1906158)
  2. Build and drop a module SDK and an APEX.
  3. Build a system image from prebuilts.
  4. See the odex and vdex files generated in
     $ANDROID_PRODUCT_OUT/system/framework/oat/
Change-Id: I8f4eaed10a1053cd560b8583efa12dc495f58db1