For prebuilts, use the built_module for nostripping dexpreopt.

Earlier, we would use the prebuilt directly, which doesn't go through
build optimizations like uncompressing the dex files.

bug: 63920015
Test: m -j32 && verify priv-apps prebuilt's vdex don't contain
      the dex code.

Change-Id: I1a69d3b6832e3a940616f71a95c13091a4220b4b