Re-enable media platinum tests after tradefed fix

Move media platinum tests from staged-platinum-postsubmit
back to platinum-postsubmit

Bug: 140253805
Bug: 157014488
Test: observation of staged-platinum-postsubmit dashboards
Change-Id: I956b615586ff5df2fbf9367bc93f4c5740442174
diff --git a/media/TEST_MAPPING b/media/TEST_MAPPING
index 206f87f..b006f38 100644
--- a/media/TEST_MAPPING
+++ b/media/TEST_MAPPING
@@ -48,7 +48,7 @@
         }
     ],
 
-    "staged-platinum-postsubmit": [
+    "platinum-postsubmit": [
         // runs regularly, independent of changes in this tree.
         // signals if changes elsewhere break media functionality
         {