Revert^2 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: atest audiorouting_tests
atest audiosystem_tests
atest audiopolicy_tests
(with lunch target aosp_cf_x86_64_only_phone and
 aosp_cf_x86_64_auto migrated on cap engine)

Change-Id: I601d95e02677c2ae82f2d03ec0b0d7fd2e3eb65a
Signed-off-by: François Gaffie <francois.gaffie@ampere.cars>