Let DEXOPT_IDLE_BACKGROUND_JOB select dalvik.vm.background-dex2oat-*

system properties in ART Service.

This code path isn't accessible, but this is done for the sake of
completeness in case that changes.

Test: adb root
      adb shell setprop dalvik.vm.background-dex2oat-threads 3
  then make a call to DexOptHelper.performDexOpt with appropriate flags
  and dalvik.vm.useartservice=true, and check that dex2oat is called
  with -j3.
Bug: 251903639
Change-Id: I8580b8974aada31969e93197ff8be09a045c3354