Update voice interaction icon shadow

Use MD style shadow, also refactored how MD shadow values
are created.

BUG=b:35624617
TEST=build and flash locally, saw the shadow changes

Review-Url: https://codereview.chromium.org/2951583002
Cr-Commit-Position: refs/heads/master@{#481330}
4 files changed