* Add 'snippet' column in Message table and handle upgrades to the
new schema
* Generate a snippet from either HTML or plain-text message body,
removing tags, extraneous whitespace, and other superfluous text
along the way. Store the snippet in the Message table
* Clean up MessagesAdapter to use the pre-existing list projection
and constants
* Write unit tests for snippet creation
* The UI in this CL is always single-line, ellipsized
TODO: Handle two-line subject if portrait and XL
Change-Id: I84a2cbe10957975942edad6eb1255a726924a78a
