Use full day names in the a11y version of schedules in mode tile subtitles

This also moves getDynamicDescription() off ZenMode to a helper class, where it should've probably been all along.

Bug: 370358575
Test: atest ModesDialogViewModelTest ZenModeDescriptionsTest
Flag: android.app.modes_ui
Change-Id: I03e98e2e95f8567347f8a246cd1953707bddb570