Updates to new back indicator

 - Refactor EdgePanelParams to make use of inner classes to simplify
   repeated fields.

 - Fix inconsistent resources names.

 - Move logic for calculating stretch amounts into the BackPanel view.

 - Re-use the existing navigation_edge_action_progress_threshold dimen.

 - Use a minimum appearance duration for calculating the pause duration
   for fling animations rather than pausing for a fixed duration.

 - Animate arrow out with an alpha fade when it is cancelled.

Test: Enable predictive back animations in Developer Options
Bug: 217579545
Change-Id: Ifb0c34a4ad892981072e0df84685e00cd3ace8ce