Make suggestions header sticky in widget picker two pane

The final design makes the suggestions header sticky, but the implementation has it following the original design which wasn't sticky. This CL makes the header sticky.

Bug: 277163859
Test: Verify that suggestions header is sticky in both 1 profile view and in work/personal profile view
Change-Id: I795617edc6d5ba410a97ff87385ec40a62abbc3e