Instead of the number of automatic modes, simply list the first (up to 3) existing modes, according to the standard sort order, when none are active.
As a consequence, the summary is never empty now.
Fixes: 374179941
Test: atest ZenModeSummaryHelperTest + manual
Flag: android.app.modes_ui
Change-Id: I04f9d90a5e1e4ed13d66181b85d77fef8c6ff256