Fix getParameters() default implementation.

Do not return empty string but key value pairs with empty values.
diff --git a/libs/audioflinger/AudioHardwareStub.cpp b/libs/audioflinger/AudioHardwareStub.cpp
index 1a03059..ae391ee 100644
--- a/libs/audioflinger/AudioHardwareStub.cpp
+++ b/libs/audioflinger/AudioHardwareStub.cpp
@@ -152,6 +152,12 @@
     return NO_ERROR;
 }
 
+String8 AudioStreamOutStub::getParameters(const String8& keys)
+{
+    AudioParameter param = AudioParameter(keys);
+    return param.toString();
+}
+
 // ----------------------------------------------------------------------------
 
 status_t AudioStreamInStub::set(int *pFormat, uint32_t *pChannels, uint32_t *pRate,
@@ -187,6 +193,12 @@
     return NO_ERROR;
 }
 
+String8 AudioStreamInStub::getParameters(const String8& keys)
+{
+    AudioParameter param = AudioParameter(keys);
+    return param.toString();
+}
+
 // ----------------------------------------------------------------------------
 
 }; // namespace android