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
AOXP: tweaks

Play Store 4.9.13
GMS 5.0.89

Change-Id: I9f0b0bee3a7a533027a878a406b32f9bd4cd4c25