Widget picker two pane polish

-increase width of two pane picker
-center widgets when they're too few to overflow
-make background stretch out all the way to the bottom even when there aren't multiple widgets
-fit many widgets in one row if they fit

Bug: 256684299
Test: open widget picker and notice that all the listed fixes match the last figma added in the bug
Change-Id: Id4e8445f4ffe484c24c04fd9d1b4ffbf93a00bb2