audio: Fix fixed size char array conversions

For legacy HAL strings that are fixed size arrays
the conversion code was using the array size for
the HIDL string size. This lead to logging of error
messages during reverse conversion.

Fixed issue and refactored code to avoid duplication.

Bug: 181269159
Test: atest android.hardware.audio.effect@7.0-util_tests
      also, verify that no error messages from EffectUtil
      appear during boot and audio playback
Change-Id: Iac36ff33e65c502966ac2b7a4870cb5830545b23