[RenderScript] Update RenderScript support lib documentation.

Bug: 23159764
Bug: 26862970
Bug: 28177406

  - Update documenation of Allocation.setAutoPadding and copy APIs as
    go/ag/903325.
  - Update documentation of ScriptIntrinsicBlur as go/ag/28177406.
  - Remove unused imports in Allocation.java

Change-Id: If1eaa0bffa8c694947b80fd40b39ff61ce8e10d0
(cherry picked from commit b139bfeb57c3bb2b2042da201e5314c3f6fcd13f)
2 files changed