[Feature/Improvement] Prompt for confirmation when exiting a timer

**Purpose:** As a user, I would like to be prompted for confirmation when hitting the back button to leave a running timer.

**More details:**

Shown in the screenshot below is the back button on the workout screen.


Currently when the user taps this button, the workout is immediately cancelled and the user is sent back to the previous page. Tapping this button could happen by mistake and the user does not want to cancel the workout. There should be an [AlertDialog](https://api.flutter.dev/flutter/material/AlertDialog-class.html) that appears when the user taps the back button, asking them if they are sure they would like to exit the workout.

The dialog should read:

**Exit Workout**

Are you sure you would like to exit the current workout?

..............................Return to workout ......... Exit workout

AlertDialog implementation will be similar to the one in #107

**Acceptance Criteria:**
- [AlertDialog](https://api.flutter.dev/flutter/material/AlertDialog-class.html) implemented and appears on back button press on the workout screen.