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This adds "Learn more" link on
Settings > Device > Storage management > External storage preferences
for detailed explanations about the setting.

Bug: 1007188
Test: Manually tested on kevin.
Change-Id: I4774bc1051e32c994e937fafadc017f9196295de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1820922
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702327}