Fix scrolling bugs in full widgets sheet

Bugs:
1. Layout margins were not taken into account
2. Fast scroller offset has a different scale to the recycler view
3. Update padding & margin in onMeasure rather than in onLayout

TODO: fast scroller measurement doesn't take into account of
      expanded item. It naively use the same height for all items
      from the recycler view adapter.

Test: Open full widgets sheet for both work profile and non-work
      profile setup. Scrollbar initial position is correct. Scroll
      up and down to confirm views vertical translation is correct.

Video: https://drive.google.com/file/d/1kp3iSm23RVk_otBNeYalEPPkWjjWMOY0/view?usp=sharing

Bug: 181629430
Change-Id: I398a94510751782e78aa8f426f37b03ecca8ec99