Etar: Define path to assets directory

Soong expects the assets directory to be in /assets, so it was not
being included in the apk. That resulted in breaking the timezone
selecting in Etar settings. This is not an issue for gradle builds,
as that expects the assets to be in /app/src/main/assets.

Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/5045
Change-Id: I95cd6815eed2bc72e92e6695112bceb1503c6199