[BottomSheet] Fix velocity-based dismiss through nested scroll path

[harveylx]

closes #5076

onStopNestedScroll called getYVelocity() which returned 0 because activePointerId was already reset by onInterceptTouchEvent(ACTION_UP). Store the fling velocity from onNestedPreFling and use it in onStopNestedScroll so shouldHide() receives the real velocity.

Changes

• Added lastNestedFlingVelocityY field to capture fling velocity from the nested scroll path
• In onNestedPreFling, store velocityY when the fling is consumed
• In onStopNestedScroll, prefer the stored fling velocity over getYVelocity() (which returns 0 due to the invalid pointer ID)
• Reset the stored velocity in onStartNestedScroll at the beginning of each gesture

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