Fix #2810: Added label for CompletedStoryListActivity

[ChiragKalra]

Explanation

Fixes #2810

Added label for CompletedStoryListActivity to read "Stories Completed" when talkback is on.

Created a common string resource "completed_story_list_activity_title" for all layouts of this activty.

Added test for CompletedStoryListActivity title.

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: ...".)
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[anandwana001]

PR title - Fix #2810: Added label for CompletedStoryListActivity

[rt4914]

@ChiragKalra PTAL.

[rt4914]

Nit change.

[rt4914]

LGTM, thanks.

[BenHenning]

Thanks @ChiragKalra! I have no comments to add--this LGTM!