DeviceUtils: Write checksums to cache even when not reading them
DeviceUtils: Write checksums to cache even when not reading them

This allows clients to use DumpCacheData() even when
enable_device_files_cache=False.

Useful for when enable_device_files_cache is set by a flag (so that it doesn't
require two runs to be effective)

BUG=540857

Review URL: https://codereview.chromium.org/1386353005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353066}

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