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This fixes a bug where the live caption UI appeared while SODA was
downloading, and showed an error message. This CL has the caption
controller observe the SODA installer and only creates the UI when the
installation has completed, or if SODA is already installed.

Bug: 1055150, 1160272
Change-Id: I32bd21344c88e6bcf1c79399e52ce994254f50ef
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595841
Commit-Queue: Abigail Klein <abigailbklein@google.com>
Reviewed-by: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/master@{#839763}