Update two pane sheet widget picker to use RelativeLayout since...

Update two pane sheet widget picker to use RelativeLayout since ConstraintLayout was causing test failures

There are test failures when using ConstraintLayout without a known solution, see https://chat.google.com/room/AAAACwktJi8/2OogGWfbLvI. Switching back to a relativelayout to resolve those issues.

Bug: 301918659
Test: WidgetPickerImageTest
Flag: ACONFIG com.android.launcher3.enable_unfolded_two_pane_picker DEVELOPMENT
Change-Id: I004c2d77ca308d85b29217d4f1e548067126556f