Prevent multiple Boca SWA launch operations

This change updates the Boca SWA launch helper to ensure there is at
most one launch operation in progress. This will prevent any race
with tab management operations.

Bug: b:377333184
Change-Id: Ia5d243800277277713f48bf5f206308a994575e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6001893
Reviewed-by: Matthew Zhu <zhumatthew@google.com>
Reviewed-by: April Zhou <aprilzhou@google.com>
Commit-Queue: Vignesh Shenvi <vshenvi@google.com>
Cr-Commit-Position: refs/heads/main@{#1379351}