audiopolicy: cap engine: fallback on default configuration

As default engine, allow to fallback on default config if the Cap Engine
could not find its top configuration file.

Bug: 379596346
Test: m

Change-Id: I24447bb818e0753e8d37b4c28a7e53e0c32bcda4
Signed-off-by: François Gaffie <francois.gaffie@ampere.cars>