Added Accessibility Support for Screenreaders

[ish-joshi]

PR Checklist:

• I have tested this extensively and it does not break any existing behavior.
• I have added/updated examples and tests for any new behavior.
• If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]
  

PR Description

The PR adds support for screen-readers. The chart can now be detected and a description is generated dynamically for bar, line and pie charts common use case scenarios.

Previously, the charts were just ignored by the screen-reader. This is a major improvement and paves the way forward for better accessibility implementation for this amazing library.

Adds autogenerated description for screenreader as well as the ability for the user to add accessibility description which will be read out when the screen-reader focuses on the chart.

chart.setAccessibilitySummaryDescription("This is a customizable description on user end");

Accessibility is really important, 2.2 billion users have some sort of vision-impairment according to the WHO. This enables access for those sets of users who cannot see and makes the library charts accessible.

[hannesa2]

If you really need this, you should continue here

[hannesa2]

It's merged in AppDevNext/AndroidChart#112

[PhilJay]

Thanks a lot for this pull request, and sorry it never got merged. MPAndroidChart 4.0.0 is a complete rewrite in Kotlin (https://github.com/PhilJay/MPAndroidChart/releases/tag/v4.0.0), so this change no longer applies to the code base and I am closing it. If it is still relevant for 4.0, a new pull request against master is very welcome. The Java 3.x code lives on in the 3.x branch.