[MaterialButtonGroup] Long press gesture detectors on children may not work (depending on position)

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**Description:** Long press gesture detector on a button in `MaterialButtonGroup` often fails (depending on its position).

**Expected behavior:** Gesture should be reliably detected.

**Android API version:** 17

**Material Library version:** 1.14.0

**Device:** Pixel

This is more for posterity than anything else, as I assume it won't be fixed. I have a `MaterialButtonGroup` with 3 `MaterialButton` children. Buttons at index 0 and 2 have a `GestureDetector` to detect a long press. Index 1 has weight 0, the others have weight 1.

The button at index 0 reliably detects the gesture, but button at index 2 doesn't. If moved from index 2 to 1, it works. This is because the button shape change that happens when it is pressed moves the `MotionEvent` location, such that it sometimes (not always) falls out of the detector's slop rectangle.

Other than disabling the button shape change (setting `app:buttonSizeChange` to `@null`) I haven't found a way around it.

Start with the MaterialButtonGroup entry point and reproduce the long-press behavior with three MaterialButton children, focusing on how button shape changes affect MotionEvent positions. Verify behavior for children at different positions, including index 2, while keeping button-size changes enabled; done means long presses are detected reliably.

Written by the indexing model from the issue text.