Fix a11y issues in the schedule editor

* Don't read start / end time as separate labels.
* Fix content description of day buttons (ToggleButton's stateDescription is textOn/textOff -- which are the same for this particular button, thus it wasn't possible to know whether a day was selected or not).

Fixes: 346396147
Test: manual, with Talkback
Flag: android.app.modes_ui
Change-Id: If73a791cf9bd62cf17e058c81a8051b3e7fd82ea