Refactor Gaussian blur API

- Changed the filter context creation API:

    - More intuitive function and arguments names
    - Kernel size parameters added
    - Intermediate size now automatically deduced from
      the kernel size parameters

- Changed the implementation API:

    - Unified separate kernel size APIs into one
    - Changed specification to allow future support for
      custom sigma values

- Simplified the HAL handler

- Modified unit tests and also added some new ones

- Other miscellaneous refactoring