Minor improvements on BugreportProgressService.

Such as:

- Include userId on notification log statements
- Made classes final
- Removed unused constant
- Reused notification code
- Change unit test to use mockito rule
- Other cosmetics changes in the test (like removing m prefix on
  public attributes)

Bug: 423702145
Test: atest BugreportProgressServiceTest
Test: manual verification
Flag: EXEMPT logging and tests changes only

Change-Id: Ib8d85fc3d445797f839ebbb5f888dc2a59c44d13