Fix up findSelectionHotspot and remove unused methods

When the use_material3 flag is enabled, the findSelectionHotspot
method attempts to find the `item_root` UiObject2. However it
appears like this ID isn't available. The method then ends up
traversing the hierarchy until the dir_list is found and ends
up selecting the first item in the list instead. Update that
method to instead check at each layer if there exists a hotspot
and return in that case. There are a number of methods that are
not being used either (which call findSelectionHotspot) so let's
remove those to avoid having an inflated number of callees for
this method.

Bug: 383669583
Test: atest com.android.documentsui.FileManagementUiTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ifca68eb60563ebd2b5a42ad7615ad60d49106fc4