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Add the function in InputDeviceManager to get all the uncategorized
input devices. The categorized input devices currently includes mouse,
keyboard, touchscreen and touchpad. After add this function, we can
also get the InputDevice like side volume button.

This is an initial change for side volume button control based on
screen orientation for convertibles.

Bug: 937907
Change-Id: I750e9919d47c11f8e881dc3f3db42bc0b0f1108c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559247Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649706}