email: prevent stock email app to unset firstSnippet on mark/unmark as read

If the email conversation only have one message then firstSnippet is the only valid
snippet in the converation item. Multiple conversation items are only supported by Gmail
which call the "set" method to set those snippets. This changes prevents to set
firstSnippet = null when user mark/unmark the item as read in stock email app.

Change-Id: I85f2ff0ab212c2a64450c3d46323418b9340fe40