Update fullscreen handle appearance.

Makes fullscreen desktop handle rounder and aligns its brightness with
notification icons.
Before: https://screenshot.googleplex.com/89zkkMpUefHc39L
After: https://screenshot.googleplex.com/3ceqjybE6QvHsWe

Cherry picked I4cd9a65366c2e3f17c22f54c85a64e53b50e6b9a to master due to b/286408867.

Code was merged via `-s ours` and is missing even though the sha exists on the target branch. Cherry pick is required to bring in the code.

Manually remove the "Merged in" directive to allow for downstream propagation.

Bug: 280830923
Test: Manual
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:6980cec2dd5770595e0c804083ef011f7fd81570)
Change-Id: Id1dfe3f02875155700cd1f3e32222b8a6dfbdf0e