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This adds an ANNOUNCE event type to let the flutter
bridge client use the tts engine to make announcements.

Also adds sort keys in order to override the default
ordering for nodes.  Subsequent CL will implement
the sorting.

Bug: None
Test: unittest
Change-Id: I4c0f00ad2318208854f1d14c63f34307fe2ddc22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446552Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814185}