Add Getter for App ID

(This is a merge of http://go/wvgerrit/13973)

The app ID was write-only because no getter was ever written.

Change-Id: Ie5e383ee3a2dcf2843a6671becb6ff1f88e97d24

libwvdrmengine/mediadrm/src/WVDrmPlugin.cpp

@@ -492,6 +492,8 @@ status_t WVDrmPlugin::getPropertyString(const String8& name,
     return queryProperty(QUERY_KEY_NUMBER_OF_OPEN_SESSIONS, value);
   } else if (name == "maxNumberOfSessions") {
     return queryProperty(QUERY_KEY_MAX_NUMBER_OF_SESSIONS, value);
+  } else if (name == "appId") {
+    value = mPropertySet.app_id().c_str();
   } else {
     ALOGE("App requested unknown string property %s", name.string());
     return android::ERROR_DRM_CANNOT_HANDLE;