audio: add support for stub HAL in HIDL mode

Fixed broken functionality for some devices relying on
a stub HAL module to be present (e.g Fugu).

Bug: 34360879
Test: make and check stub HAL is loaded in HIDL passthrough mode

Change-Id: Ibad050d63827c4e49604273efd76ac8074bb00d2
diff --git a/audio/2.0/IDevicesFactory.hal b/audio/2.0/IDevicesFactory.hal
index f1dc7d3..0ef6bc5 100644
--- a/audio/2.0/IDevicesFactory.hal
+++ b/audio/2.0/IDevicesFactory.hal
@@ -26,7 +26,8 @@
         PRIMARY,
         A2DP,
         USB,
-        R_SUBMIX
+        R_SUBMIX,
+        STUB
     };
 
     /*