Add outline to the transient task bar background.

Added outline to the transient task bar background.
Demo: http://recall/-/gx8ASgewUeUS3QYohfrd1J/gmJKD6VAuGXP8KOP1vLwad

Fixes: 330785041
Flag: NONE Trivial UI update
Test: Manual. Open transient task bar with the light and the dark
themes.

Change-Id: I69db4b40a1bd101471c2a2556a0621ac4494b8ec