libaudiohal@aidl: Make IHalAdapterVendorExtension mandatory

Since IHalAdapterVendorExtension is not stable, it can not be
included into the framework manifest, and thus `isDeclared`
check will never succeed (due to a bug, it could be included
previously, but it is now fixed).

This service is necessary for any vendor because all vendor
HAL implementations have some vendor-specific parameters.
An example implementation is added, which is used by Cuttlefish
and also added to GSI system_ext.

Bug: 321651892
Test: atest audiorouting_tests
Change-Id: Ib529250349da289cecb26bb965f1bfc8e37dc893