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
diff --git a/media/libaudiohal/impl/DeviceHalAidl.cpp b/media/libaudiohal/impl/DeviceHalAidl.cpp
index 2af18cc..fe44ecd 100644
--- a/media/libaudiohal/impl/DeviceHalAidl.cpp
+++ b/media/libaudiohal/impl/DeviceHalAidl.cpp
@@ -1024,15 +1024,11 @@
     (void)VALUE_OR_RETURN_STATUS(filterOutAndProcessParameter<String8>(
                     parameters, String8(AudioParameter::keyReconfigA2dp),
                     [&](const String8& value) -> status_t {
-                        if (mVendorExt != nullptr) {
-                            std::vector<VendorParameter> result;
-                            RETURN_STATUS_IF_ERROR(statusTFromBinderStatus(
-                                    mVendorExt->parseBluetoothA2dpReconfigureOffload(
-                                            std::string(value.c_str()), &result)));
-                            reconfigureOffload = std::move(result);
-                        } else {
-                            reconfigureOffload = std::vector<VendorParameter>();
-                        }
+                        std::vector<VendorParameter> result;
+                        RETURN_STATUS_IF_ERROR(statusTFromBinderStatus(
+                                mVendorExt->parseBluetoothA2dpReconfigureOffload(
+                                        std::string(value.c_str()), &result)));
+                        reconfigureOffload = std::move(result);
                         return OK;
                     }));
     if (mBluetoothA2dp != nullptr && a2dpEnabled.has_value()) {