Fix copyFrom() to use proper dimensions for copying.

This change actually fixes several bugs related to stride:

* copyFrom() needs to call the 2D or 1D version, depending on the dimensions
  of the corresponding Allocation.
* Add an internal-only copy2DRangeFromUnchecked(), since we don't really
  want to expose it as another public API (only via copyFromUnchecked()).
* Call the proper 1D/2D version in copyFromUnchecked() based on the
  Allocation dimensions.
* Add Element checks to the "checked" copy2DRangeFrom() routines.

Change-Id: I690706d36884ee749bf90937c715855f6c07368c