Set avatar visibility based on collapsed state

And not whether it's a single line because the collapsed form
can have multiple lines if a summarization is shown

Also, fix an NPE seen when testing this fix with a group without a
name

Test: manual - collapsed and expanded forms of group and individual
notifications with and without summarization
Test: presubmit screenshot tests
Flag: android.app.nm_summarization_ui
Flag: android.app.notifications_redesign_templates
Fixes: 399841390

Change-Id: I606a94275cf084e3c3da549a38b204fad3fa8f46