Fixes #47: Replaces Gravatar with Avatars with colored backgrounds

[jamesxu0]

Explanation

Fixes #47: Replaces Gravatar with Avatars with colored backgrounds. I had to create a new PR because I accidently merged the old PR #539 into my own branch instead of develop.

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.
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• The PR is made from a branch that's not called "develop".
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[seanlip]

Approving based on @BenHenning's approval of #539. There are three slight differences from that PR in import statements and ordering (see app/src/main/AndroidManifest.xml, app/src/main/java/org/oppia/app/profile/AdminSettingsDialogFragmentPresenter.kt) but the current version looks fine to me.