conradchen 3db25be3b6 [ButtonGroup] Simplify MaterialButtonToggleGroup's checking logic
1. Consolidates single selected ID and multiple selected IDs to a single selected
   ID set.
2. Separates View states and internal checked states so we can focus and enforce
   policies much easier on only internal states.

PiperOrigin-RevId: 420087718
2022-01-06 15:44:19 -05:00
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2021-12-22 14:46:19 -05:00