audiopolicy: fix to play 6.1 channel ALAC clips

For 6.1 channel ALAC clips, the compress offload profile is
not found even though the channel mask is defined in the
audio_policy.conf file. This is because the channel mask enum
is not defined and hence the profile's channel masks
are not getting enumerated properly.
This change is needed for 6.1 channel ALAC clips playback.

Change-Id: I6b820776c0dc6e68a402886f0931439edab24a8b