SDK / exoplayer

These flags ensure that any errors cause the script to exit (instead of
just carrying on) (`-e`) and that any unrecognised substitution variables
cause an error instead of silently resolving to an empty string (`-u`).

Issues like Issue: google/ExoPlayer#10791 should be more quickly resolved with
`set -e` because the script will clearly fail with an error like
`make: command not found` which would give the user a clear pointer
towards the cause of the problem.

#minor-release

PiperOrigin-RevId: 490001419
(cherry picked from commit f8344197)