mark mPixelStorageType const

This is a natural consequence of the fact that we always call
setPreLocked from the constructor. Future Skia will remove the virtual
for lockPixels (onNewLockPixels), but that is compatible with this
subclass since does not change its pixel-addr or pixel-type after the
constructor.

See skbug.com/6481 for more details

Test: existing tests apply, no behavior change

Change-Id: I0f9d3efc9a05b71907480eaaadbc7fafe9dd491b