In HdrViewfinder using macros and proper configuration

In documentation about RenderScript there is information about creating kernels with using macro `RS_KERNEL` in example code there is ugly `__attribute__((kernel))`.

Second item, in documetation about RenderScript there is information about configuring build.gradle:
```
renderscriptTargetApi 18
renderscriptSupportModeEnabled true
```
Why there is no such entry in example config?

Start with the RenderScript documentation and the HdrViewfinder example referenced in the issue. Compare the kernel syntax and the sample build.gradle configuration with the documented guidance; done means both examples are consistent and the expected RenderScript settings are explained or included.

Written by the indexing model from the issue text.