Setup Oppiabot stale checks for Oppia Android

[jameesjohn]

Explanation

This PR sets up oppiabot stale checks.
It will mark PRs without updates for 6 days as stale, and close them after 4 days.

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.

[jameesjohn]

PTAL @BenHenning

[BenHenning]

Thanks @jameesjohn! Just had one comment--PTAL.

[jameesjohn]

Hi, @BenHenning I responded to your comment, PTAL.
Also, how do I fix the failing test? It seems like a code owners check.

[BenHenning]

@jameesjohn apologies for not following up earlier--I tend to only look at assigned PRs since those require my active attention.

To fix the codeowners check you'll need to add the new file to CODEOWNERS (making me the codeowner here should be fine).

[jameesjohn]

Done.
PTAL @BenHenning. Thanks

[BenHenning]

Thanks @jameesjohn! This LGTM--I appreciate you making this config change for us.