Adding back button in settings activity and confirmation dialog in main activity

[htg2704]

Closes #372
Closes #369

What has been done to verify that this works as intended?

Verified on a physical device.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Better user experience.

Before submitting this PR, please make sure you have:

• run ./gradlew checkCode and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.

[huangyz0918]

Please resolve the CI conflicts and remove the unnecessary changes for the #372. Or you can open a new PR for #367 . But as for me, adding an exit button to settings activity is not a necessary change, but if you want, I'm ok.

[ajay-prabhakar]

@htg2704 are you still working on this?

[htg2704]

@Chromicle I made a commit 20 days back. I will see why it's not passing the test because according to me I made all necessary changes.

[ajay-prabhakar]

yeah, but you build is failing due to pmd check, run ./gradlew checkcode locally you will get detailed information on how to solve that error
and please solve the conflicts