audio: Use strings for EffectDescriptor.{name|implementor}

The use of fixed size char arrays was a direct copy
of the C structure approach. In HIDL, the canonical approach
is to use strings.

Bug: 181883090
Test: atest android.hardware.audio.effect@7.0-util_tests
Change-Id: I468810e07e5ba8b3bd6f03c5acfa24009ef2e48a