Nodes with contentDescription should always be important for accessibility.

1. Now after setting the content description on a view we mark is as
   important for accessibility of the current important for accessibility
   mode of that view is auto.

2. Minor tweak to a touch explorer coefficient to make performing double
   tapping easier.

bug:6615353

Change-Id: I3b477f533a3ebde85d425caf32ace5e851240f88