[Launcher3] Hide keyboard when fastscroller scrolling down and keyboard

shown

Implement keyboard hiding logic: Hide keyboard when scrolling down (y > mLastY) unless already requested (mRequestedHideKeyboard). Track hide request state to prevent redundant calls.

result video: https://photos.app.goo.gl/e4dYUUiUN3BujNUz9

Bug: 333967597
Flag: EXEMPT bugfix
Test: Manual
Change-Id: I42b9d05dd5368b021577f12541248d434ee22495