audio policy: fix combo device parsing

When parsing a device type entry in audio_policy.conf, do not
load device combos (e.g AUDIO_DEVICE_OUT_ALL_A2DP) in a single
DeviceDescriptor but create one descriptor for each device type
in the combo.

Bug: 24113912.
Change-Id: I3a7f43c7fc9cf516506bb3255dd2ed86564b7cd6