create_app_bundle.py: Process splits in alphabetical order

Rather than add them to the bundle in the order they appear on the
command-line, add them in alphabetical order (but with "base" coming
first).

Do this in order to have build artifacts not depend on argument order
(less chance of refactorings changing build artifacts)

Bug: None
Change-Id: Ifa8ab4b45ffa05b7dc7f5ffb3d5ef4fd4c0ca040
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4470189
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1134870}