Fix null pointer dereference in Soong on dexpreopt arc-services.

The null pointer dereference was caused by trying to get device name
from a config passed to dexpreopt. For Make modules, the config is a
stub that doesn't have a device name. The fix is to use build directory
instead of device name (it is provided both by the full config passed
to Soong modules and by the stub config passed to Make modules).

Test: walleye-userdebug boots.
Test: aosp_bertha_arm-userdebug builds and generates arc-services.odex.

Bug: 152193534
Change-Id: I7adc173646cae6eaee188c6672629a5a942aea06