Audio: Fix Visualizer's get/set parameter methods not worked bug

This commit fixes broken Visualizer's get/set parameter methods.
The bug was introduced by modifications to avoid complilation errors.(*)

(*) b9401ac19ac2c238bf4be006dfded1408dd458f8

    Fixing aliasing violations, C++ 11, and gcc 4.7 changes from linaro.

    av: Fix duplicate definition of size_t n

    Needed to make it build with gcc 4.7+