Fix #862: Mark chapter completed

[rt4914]

Explanation

Whenever an exploration is finished we need to save its progress as COMPLETED, this PR does that only.
Test cases should be added for this functionality but after having discussion with @BenHenning it was decided that we will introduced test cases for this once #495 is merged into develop. Issue for this is #880

For testing purposes you can take help from here.

1. Uninstall the previous app.
2. Install the latest copy of app.
3. You will see default recommended explorations in promoted stories.
4. Now lets finish first exploration in Fractions. Answers are mentioned below:

• Three Fifth
• Four Sevenths
• Second Option
• 2/7
• Second Option
• 1/6
• 5/6
• Second Option
• 4/9
• 8
• 9
• First and Third
• 3/10
• 4/6

5. Now mark exploration as finished. Close the app. Run app again, you should see Meaning of Equal Parts as promoted story which is the second exploration in FRACTIONS.

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The PR explanation includes the words "Fixes #bugnum: ..." (or "Fixes part of #bugnum" if the PR only partially fixes an issue).
• The PR follows the style guide.
• The PR does not contain any unnecessary auto-generated code from Android Studio.
• The PR is made from a branch that's not called "develop".
• The PR is made from a branch that is up-to-date with "develop".
• The PR's branch is based on "develop" and not on any other branch.
• The PR is assigned to an appropriate reviewer in both the Assignees and the Reviewers sections.

[veena14cs]

LGTM.