vendor/omni: Fix-ups for prebuilt Chromium

- Make copying far more reliable, even on non-clean builds
- Use $OUT in script in all cases, apparently get_build_var OUT_DIR isn't reliable.
envsetup.sh, please -_-

Change-Id: I27fa9bc7a043c32bcb29e7667e86b40226222a5a
Merge pull request #4 from omnirom/android-4.4

vendor/omni: Fix-ups for prebuilt Chromium