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Needed for things like audio and audio effects. Provides a
new interface to loading modules named 'hw_get_module_by_class'.
This takes two parameters: 'class_id' and 'instance' which are
used to construct the filename for the module to be loaded. If
instance is NULL, then this function acts identically to
hw_get_module where 'class_id' == 'id' (and in fact the latter
implemented exactly this way).

For audio, this new mechanism allows us to load multiple audio
interfaces by doing:

hw_get_module_by_class("audio", "primary", &module);
hw_get_module_by_class("audio", "a2dp", &module);
hw_get_module_by_class("audio", "usb", &module);
...

In the future we will likely want to add the ability to load a set of
module instances based on a config file, which will have a standard
syntax and the mechanism will be provided by libhardware.

Change-Id: I9976cc6d59a85a414b18e7b398a36edfbce4abd8
Signed-off-by: Dima Zavin <dima@android.com>