Include % synced in metrics + other metric calc tweaks.
Also fix bugs in the non-Gmail IMAP metrics updating.

Summary:
Monocle would now display the following metrics:
# Remote, As of, % synced, # remaining, # downloaded, As of

Test Plan: Visual app inspection.

Reviewers: spang

Reviewed By: spang

Subscribers: charles

Differential Revision: https://review.inboxapp.com/D299