Add Scatter Chart to statistics fragment

[chidauri]

Closes #178

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

Tested on Android 9, sending and receiving different forms.
Screenshots:

Why is this the best possible solution? Were any other approaches considered?

This is a minimalistic graph, which does not bloat the screen too much at the same time conveying the required information.
I've chosen to show past 30 days activity, no more than that, because

1. I believe Statistics are less relevant after a month
2. On portrait view, we have to minimize the dates we can show.(Currently only the current week is visible and user would have to swipe left on xAxis to see more.)

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?

Users who want a more detailed statistics of the forms, would have to select Show Detailed Statistics checkbox in General Settings, the scatter chart is programmed to show past 30 days activity of that instance.

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]

@chidauri Thanks for your contributuon!
But I think #178 is not aim to repeat showing the same data in another kind of chart. For scatter chart, that is great, but in data science as I know, scatter plots are used to observe relationships between variables.

Do we have any relationships between send/receive/review data? For showing the number of those data, I think bar chart is the best and enough. Maybe we should explore more kinds of data that we can provide and present them to our users.

[chidauri]

@huangyz0918 Hi, as mentioned in #178 (comment)
the task was to show Sent/Received/Reviewed Instances of a form per day, maybe ScatterChart doesn't suit this requirement, we can try Grouped BarChart if you say, which will look similar to this.