Hide read-only accounts from side nav

* Load writable accounts only so we don't need to remove read-only accounts.
* We don't need to add local account in side nav for now, because:
  - we only show writable accounts in side nav
  - and we add local account only when there's no writable account
  - and we don't show any account in side nav when the number of accounts
    to display in side nav is smaller than 2.
* Revert the lines to filter accounts (line 139-141) to the status before
  ag/1198595.
* With this CL, Skype account will not show up in side nav because it's
  read-only; however, the label that belongs to Skype still shows up in
  the label section of side nav (see b/29046877).

Bug 30707025

Change-Id: I3337894880db2ad787ab42fe7a10b29371ed5f16