Respect user restrictions about adding/removing accounts, sideloading

Hide or disable options in the settings app when the user is restricted from
making changes.

Remove "Add account" button from Settings menu, and "remove account" option from menu in AccountSyncSettings.
Remove sideloading checkbox in SecuritySettings.

Also handle replacement of UserManager.isShareLocationToggleAllowed() with hasUserRestriction, which takes a restriction key string.

Change-Id: I34c74fd5aed8956ba00f92e3d3c657b608454dfe