Add backstack tag for assistant helpers

Fixes a bug in Android O. Pop the helper tag and then pop the second
level tag afterwards. Let super.onBackPressed handle the singular
case of third level.
Unsure why doing it in two steps fixes the problem but it should
effectively be the same.

Test:
Manually verified navigation situations on Nexus 5X running O,
Nexus 6P running 7.1, Nexus 5 running 5.1.1, and Samsung
Galaxy S7 running 6.0.1:
  * all contacts to suggestions and pressing back
  * all contacts to suggestions to group and pressing back
  * all to group and press back
  * all to group to suggestions and press back
  * all to suggestion to duplicates press back twice
  * mixes of the above with rotations and going to a specific
    account view, verifying the list is displayed properly
  * the above but using the Contacts menu item instead of pressing
    back

Bug:33922405

Change-Id: Idb9421be79038b112eb0012e8be17e5692935c70