Declare audio_devices_t as uint32_t & enum consts

Declare audio_devices_t as uint32_t to permit bit operations,
and a separate anonymous enum for the constants.  This is similar to
audio_channel_mask_t.

Also remove spurious "typedef" on declaration of anonymous enum for
channel mask constants.

Change-Id: Ib4f92e9168261d44a475b3d48fe211e5c70ed441