Fix some taskbar stashing issues

- Don't allow long press to stash between taskbar icons
- Ensure taskbar_icon_touch_size is respected; previously it wasn't, because BubbleTextView@shouldIgnoreTouchDown() was returning true in the padding region. For taskbar, we want the whole icon size to be touchable.
- Cancel long press when passing touch slop to avoid swipe down being detected as long press

Test: long press on taskbar background, both between icons and not; swipe down on taskbar
Fixes: 198305464
Change-Id: I36f1d792e91da9a3bf57a2bef1e974b299c4e25c