[Example] Add exit when naviframe is empty

[Example] Add exit when naviframe is empty

**Changes proposed in this PR:**
- Add and use _on_back_pressed_cb when back button is pressed
- Change `view_routes_to` signiture to return naviframe item.
- Delete unused functions

**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>