Include rowContentBindStage mock in NotificationEntryManagerTest.

This test is flaky because sometimes it succeeds in inflating
its ExpandableNotificationRow, and sometimes it doesn't. When it
does succeed, it is currently getting an NPE. This CL fixes the
NPE, as one of the methods that needed mocking was missing.

Bug: 149391269
Test: atest SystemUITests
Change-Id: Ic869c0badba3db619b42a781194dfbdb4ee1a59b