Edits to strings per UI text standards.

We've been finessing some of these standards.
* Use ending punctuation for notification messages, and other messages, if a complete sentence.
* For confirmation messages needing a title, ask the question in the title and put the additional information in the message body.
* Use "choose" instead of "select" when asking the user to pick from options.
* Use "syncing" instead of "synchronizing".
* Don't use ending punctuation for setting summary text.
* Use contractions whenever possible.
* Try not to use the bang!
* Don't use "fail" in user-facing messages.

Bug: 5165745
Bug: 5163232
Bug: 5162667
Change-Id: Iafb9f8d7e93d11f2a0b04e9886bedb8981f62ce9