Implement strategies for reducing caption flickering

This CL implements a Wait-K approach[1] and a Greedy Text Stabilization algorithm[2] to reduce the amount of flickering when translating partial speech recognition results. These approaches are placed behind two separate feature flags and only apply to translations. The following table illustrates the flicker rate for a 30 second audio clip translating English to French:
        Trial 1.   Trial 2.  Trial 3.
None    24         25        26
Wait-K  16         20        20
GTS     4          4         4

[1] arxiv.org/abs/2005.08595
[2] go/w2t

Bug: 1471300,1369737
Change-Id: I288a43e411ef3bebf09aaa05346c778c7621d321
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4852478
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/main@{#1201219}