Multipaste: Add keyboard shortcut icon inline with text.

This CL adds text to the nudge view and an inline icon to represent the
key to press to activate the keyboard shortcut. The icon shows as either
a search icon or a launcher icon, depending on what the device supports.
If the launcher icon is to be shown, then the icon is changed depending
on if assistant is available.

Bug: 1105541
Change-Id: Ieaad19c899af545f38410350ece2d30a48409a11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463869
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816375}