[RenderScript] Update documentation about API level and RS Support Lib.

Bug: 22767853

  - Correct the description about renderscriptTargetApi: Use the lowest
    API level providing all the functionality in the code.
  - Update the sample gradle config to use the up-to-date and stable
    version of Build-Tools.

Change-Id: Ie8008acafb270fda9e2c928953568461f6f6f476