Abstract away access to audio devices HAL

In this CL all direct access to audio_hw_device_t and its functions is
encapsulated within the new class DeviceHalLocal. Loading of hardware
modules is encapsulated withing DevicesFactoryHalLocal.  AudioFlinger
uses interface classes DevicesFactoryHalInterface and DeviceHalInterface
to access these functions.

Bug: 30222631
Change-Id: Ic88b20c55813a24b898f4a832e082c17d81935b7