Move plugin loading related methods to PluginLoader.

Fixed all tests and added more tests to plugin loader.

Change-Id: I97cb3779117d9b29b40e1107e39d6d1c953fcfdf

polygerrit-ui/app/elements/plugins/gr-settings-api/gr-settings-api_test.html

@@ -48,7 +48,7 @@ limitations under the License.
       let plugin;
       Gerrit.install(p => { plugin = p; }, '0.1',
           'http://test.com/plugins/testplugin/static/test.js');
-      sandbox.stub(Gerrit, '_arePluginsLoaded').returns(true);
+      Gerrit._loadPlugins([]);
       settingsApi = plugin.settings();
     });