Change collapse to use adapterItems instead of getting the childCount()

Using childCount() doesn't work since it only shows what is visible on the screen.
This is an issue because if the header was somewhat slightly off screen, it wouldn't scroll up upon
clicking the button.

bug: 299294792
Test:
before: https://drive.google.com/file/d/1T4rvfR3_rNmR8tRZheBXNbYjCrSubmy7/view?usp=sharing
after: https://drive.google.com/file/d/1T4UdddM7MH4onEfNdyhDM-RowKHfvjop/view?usp=sharing
Flag: ACONFIG com.android.launcher3.Flags.private_space_animation TRUNKFOOD
Change-Id: I209e3ec707bfbdc2ef5e98034149459286903854