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This CL adds a feature flag for stylus battery
status in the stylus tools menu.
Also adds skeleton code to set up the UI to be
hooked up with backend CL in a subsequent CL.

Bug: 1158537
Change-Id: I873c1dac4827a64e0f66a18aa63a4df1e0c44036
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590366
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837886}