Fixes part of #16: Implementation of PinPasswordActivity

[jamesxu0]

Explanation

Fixes part of #16: Implementation of PinPasswordActivity. Also, it implements DialogFragments to allow the user to change their PIN. Note the base branch is not develop but refers to PR #327.

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.
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[jamesxu0]

@BenHenning PTAL. I addressed the comments from your first review.

[BenHenning]

Generally LGTM. Left a few comments--please resolve this. Otherwise, primarily deferring to Rajat's review. I didn't notice anything particularly concerning after a quick pass over the PR.