Fix Battery Usage graph layout for small screens

In PowerGaugePreference, if there isn't an icon to show, we were just
adding an empty one, which causes space to be reserved and isn't what we
want on small screens.

Bug: 111318404
Test: visual (set display size to Larger or Largest, and visit Battery
page in settings; the "Last full charge" and Screen usage since full
charge" items at the bottom should not be indented)

Change-Id: I60d39836ab7d50234022a377ff6d97dde8d4c7db