Update summary of Modes entry in Settings

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