Fixes memory leak and NPE crash issue (2/3).

We were incorrectly depending on the navigator in the view-model. The
view-model is long-lived and used across fragment/activity lifecycles.
The navigator is the fragment itself. It's not correct to keep a
reference to it in a long-lived object like the view-model. This leads
to memory leaks and to NPE crashes when attempting to touch the "more
wallpapers" button after changing the wallpaper at least once.

The approach is to remove the reference from view-model and move the
navigation logic out into the binder and section controller.

Bug: 262924643
Test: manually verified that changing the wallpaper and then clicking
the "more wallpapers" button no longer crashes

Change-Id: Ib564d1afcf346710d1ab116596336ccad5accbde