Touch Point detection algorithm should account for margins and empty areas

[basshelal]

Currently the touch point detection algorithm used when drag is started checks only if the touch point is over a valid ViewHolder that is not the currently dragging one, it ignores margins and empty areas. See the gif below:

This needs to be changed so that when the touch point is in the bounds of (margin and padding included) another ViewHolder that is different from itself, it should trigger the swap. This means if callers have VHs with large margins or padding the swaps will still work as expected.

This also means that when a drag is happening over an empty area of a BoardList, like if it is empty or has few items (and thus an empty space at the bottom) swaps won't be triggered until the touch point is over another ViewHolder. This an old issue from Waqti that was hackily solved quite poorly before but we need to properly solve this here in a scalable manner.

[basshelal]

Update: This has mostly been done, just needs some cleanups and fine tuning but it's essentially done.

• Touch detection now takes margins into account.
• Whenever touch point is over an empty area (such as over an empty list or empty part of list) the last ViewHolder will be correctly returned.
• Whenever touch point is outside the bounds of the list it's dragging over, the first or last ViewHolder is returned. So for example when dragging to the very top of the screen, before it wouldn't correctly do any swaps, now we always return the first visible ViewHolder and thus the swap is always made with that ViewHolder.

I won't close yet because it may need some cleanups, especially when we make a new ItemAnimator, but the issue is essentially fixed!