fix MediaDrm.isCryptoSchemeSupported(uuid)

1. Don't expect plugins to support an empty mimeType in isContentTypeSupported
2. Move the cts test mock drm plugin to the cts tree so it is always used

b/10528466

Change-Id: I6023f6165b1e9d294986f7e5cd0896e056e376f1
diff --git a/media/libmediaplayerservice/Drm.cpp b/media/libmediaplayerservice/Drm.cpp
index 4b527d0..eebcb79 100644
--- a/media/libmediaplayerservice/Drm.cpp
+++ b/media/libmediaplayerservice/Drm.cpp
@@ -222,7 +222,11 @@
         }
     }
 
-    return mFactory->isContentTypeSupported(mimeType);
+    if (mimeType != "") {
+        return mFactory->isContentTypeSupported(mimeType);
+    }
+
+    return true;
 }
 
 status_t Drm::createPlugin(const uint8_t uuid[16]) {