Clean up some logic around long press on nav bar buttons

- Only set the long click listeners on the buttons if we are in
  screen pinning mode since we no longer have direct actions for
  long pressing back or recents individually.

Bug: 145180234
Test: Verify talkback does not indicate long press when not in screen
      pinning and notifies long press when in screen pinning.  Also
      verify with screen pinning on/off without accessibility.

Change-Id: If86733bb00e30a285663583334b7b9b2cf707563