Rename audio_utils/conversion.h to mono_blend.h

There is only one function declared in that header file, and it is
mono_blend, so a more specific filename may be easier to understand.

Test: compiles OK
Merged-In: Ie2aec6c706b1822581b34d3702818b8200a1db8a
Change-Id: Ie2aec6c706b1822581b34d3702818b8200a1db8a
(cherry picked from commit 6bf707f4)