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
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Automatic translation import

Change-Id: I04e17aa4be7f44938e2563902f85eceee563a575
UnifiedEmail: Update for TRDS Theme

credits to kroz

Change-Id: If02cbfb6ff9e4671fc5f3ddfdbf61dab70e09ac3