libaudiohal: EnumConverter renamed to EnumBitfield

EnumConverter scope was reduced to just convert to/from bitfield
and its underlying type. This allows for operator= conversion support.

As part of this rework and to better express the new class behaviour,
it was renamed as EnumBitfield.

Test: compile
Bug: 118203066
Change-Id: I2dc108cb3e1757e685490cae978cb62cbf276423
Signed-off-by: Kevin Rocard <krocard@google.com>