Some cleanup logging and test for back navigation

- Better logging
 - Remove method form OnBackInvokedDispatcher interface (but not the
   implementation)
 - Add test for ag/17211289

Bug: 222675481
Test: BackNavigationControllerTests#testUnregisterCallbacksWithSystemCallback
Change-Id: I6a3ae30da7e29dddda116acd8fa0c9af4b9faa6e

data/etc/services.core.protolog.json

@@ -703,6 +703,12 @@
       "group": "WM_DEBUG_STATES",
       "at": "com\/android\/server\/wm\/ActivityRecord.java"
     },
+    "-1427392850": {
+      "message": "WindowState: Setting back callback %s (priority: %d) (Client IWindow: %s). (WindowState: %s)",
+      "level": "DEBUG",
+      "group": "WM_DEBUG_BACK_PREVIEW",
+      "at": "com\/android\/server\/wm\/WindowState.java"
+    },
     "-1427184084": {
       "message": "addWindow: New client %s: window=%s Callers=%s",
       "level": "VERBOSE",