[MCKIN-10516][BB-3915] Reduce drag delay on mobile

Description

We received feedback that moving items is not intuitive on touch devices. We opened 3 PoC PRs for improving it. The #208 and #209 are rejected approaches.

This increases the sensitivity of dragging items (reduced the dragging delay). This way it will be less unusual for user to wait after holding his finger on the item:
![increase_sensitivity](https://user-images.githubusercontent.com/3189670/57171932-dba87980-6e19-11e9-968a-7f6dccf0b4bc.gif)

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    1. Testing instructions
      1. Install this XBlock.
      2. Create `Problem -> Advanced -> Drag and Drop` unit.
      3. Test this on a mobile device. I've tested this with Firefox Nightly 210331.

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Assignee

Shafqat Farhan

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Piotr Surowiec

Repo

edx-solutions/xblock-drag-and-drop-v2

Customer

Github Lines Added

7

Github Lines Deleted

2