Resized thumbnails; async; extend MatrixCursor.

When requesting thumbnails, check if their dimensions are larger
than requested, and downscale to avoid memory pressure.  Load them
async and with LruCache.

Extend MatrixCursor so that RowBuilder can offer() columns without
requiring they know the projection map.  This makes it easier to
respond to query() calls, where the remote side controls the
projection map.  Use it to handle custom projections in external
storage backend.

Update date/time formatting to match spec.

Bug: 10333418, 10331689
Change-Id: I7e947a8e8068af8a39b55e6766b3241de4f3fc16