libaudiohal: Add optional "msd" devices factory

Adding code that attempts to get an instance of "msd" devices
factory. The factory isn't yet used, but having the code in place
ensures that we would know about any changes in HIDL services
discovery protocol.

Bug: 63901775
Test: audioserver starts even if "msd" HAL service is absent
Change-Id: Ia897127ba383fc05d139b633717992a5d5a4f920