Files.app: Update toolbar layout and icons on it.

- Use pixel-perfect PNGs for icons on toolbar, instead of core-icons(SVG).
- Update hover/focus color to be visible on white and blue background.
- Change the shape of hover/focus background from square to circle.
- Some minor layout change to comply with the latest spec.

BUG=402344
TEST=manually compared with the mock.

Review URL: https://codereview.chromium.org/928353002

Cr-Commit-Position: refs/heads/master@{#316555}