Added LongestPalindromicSubstring.java to Java/strings

[AzadN]

@shellhub this is the final file out of the large PR #1990 mentioned earlier. This is an O(n^2) implementation of Longest Palindromic Substring problem. It'd be really nice if you could review all my individual PRs ASAP and merge them into your repo :)

@Kush1101 could you too review this?

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

References

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Java files are placed inside an existing directory.
• All filenames are in all uppercase characters with no spaces or dashes.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• [] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[TravisBuddy]

Travis tests have failed

Hey @AzadN,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

TravisBuddy Request Identifier: 83153b90-1edb-11eb-b069-4b84fe73ac29

[ayaankhan98]

Please add proper problem statement so that it become easy for begginers to understand. As the soul purpose of these implementations is for educational purpose.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions]

Please reopen this pull request once you commit the changes requested or make improvements on the code. If this is not the case and you need some help, feel free to seek help from our Gitter or ping one of the reviewers. Thank you for your contributions!