Long screenshots: Fix/simplify ImageTileSet.toBitmap

Bug: 179378294
Test: manual

There were subtle mistakes in the drawing which caused
bounds to fail when the image had tiles at negative
offsets. toBitmap now delegates to the drawable impl
which can already be cropped with setBounds

In addition this fixes mistakes in TiledImageDrawable
where translation is applied in the wrong direction
and cropRect applied in the wrong position.

Change-Id: Ica28e47031cb28353a49cdd657666f17115c4f6c