audiopolicy: managerdefinitions: add attribute for engine dynamic loading

This CL updates the parser to support a new attributes in GlobalConfiguration
in order to support dynamic loading of engine.
The given value shall be <<none>|default|configurable>
XSD for HAL v6.0 has been updated accordingly.

Bug: 141989952
Test: build & audio check

Change-Id: I5774597e252261d8369cd7dc3cce9bb7056e1569
Merged-In: I5774597e252261d8369cd7dc3cce9bb7056e1569
Signed-off-by: François Gaffie <francois.gaffie@renault.com>