AudioParameter: add functions for working with parameter keys

Added functions that are helpful for building parameter query
strings to be used with HAL's 'get_parameter' methods.

Test: make
Change-Id: I66a4e94d65660b04ded4abb59a89d8288056a920
diff --git a/media/libmedia/AudioParameter.cpp b/media/libmedia/AudioParameter.cpp
index 7871ede..d244a0a 100644
--- a/media/libmedia/AudioParameter.cpp
+++ b/media/libmedia/AudioParameter.cpp
@@ -83,15 +83,17 @@
     mParameters.clear();
 }
 
-String8 AudioParameter::toString() const
+String8 AudioParameter::toStringImpl(bool useValues) const
 {
     String8 str = String8("");
 
     size_t size = mParameters.size();
     for (size_t i = 0; i < size; i++) {
         str += mParameters.keyAt(i);
-        str += "=";
-        str += mParameters.valueAt(i);
+        if (useValues) {
+            str += "=";
+            str += mParameters.valueAt(i);
+        }
         if (i < (size - 1)) str += ";";
     }
     return str;
@@ -108,6 +110,11 @@
     }
 }
 
+status_t AudioParameter::addKey(const String8& key)
+{
+    return add(key, String8());
+}
+
 status_t AudioParameter::addInt(const String8& key, const int value)
 {
     char str[12];