Update home controls button on keyguard

- change scale type to fitCenter so size of the icon responds to size of
  the container
- add padding around the icon so it doesn't fill the container
  completely

The home controls button looks the same visually. This change allows
overrides for large screen devices.

Test: verify the home controls button on keyguard looks the same
Bug: 274661719
Change-Id: Ia451d9bfe7f488545df156ea71ddcc0cd916e11b