Fix regression introduced in ea9d250 where corner morph animations
in MaterialButtonGroup were interrupted.

The issue was that onBoundsChange() always skipped animations by
passing `skipAnimation=true` to updateShape(). This affected state-only
changes (like button press) where bounds don't actually change.

Restored the boundsIsEmpty logic from 1.14.0-alpha06:
- Skip animation only on first non-empty bounds (initial layout)
- Allow animations for subsequent state changes (smooth UX)

Fixes material-components#4990

…maintaining performance.

After ea9d250, corner morph animations in MaterialButtonGroup were interrupted
during state changes because onBoundsChange() always skipped animations. This
caused abrupt visual transitions when buttons were pressed or focused.

Simply reverting would reintroduce unnecessary animations during layout changes
(screen rotation, window resize), hurting performance.

Solution: Use time-based approach to distinguish change types:
- Track timestamp when state changes occur (onStateChange)
- Allow animation for 500ms after state change
- Skip animation for pure layout changes (outside time window)

This maintains both smooth state transition UX and layout change performance.

Fixes material-components#4990