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This CL is a refactor with no intended behavior change. The goal is to
reduce redundant strings across different ash consumers of the
platform_keys api.

Bug: 1164523
Change-Id: I4b34cf98ac7303b7dab4e7a799b045972b61ba6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2630430
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843838}