Fixes 'capture more' on Settings home page

This change excludes the contents of 'main_content_scrollable_container'
from scroll capture search.

Explanation: The inner-most child is selected whenever nested scrolling
views are found. This is incorrect for Settings because the inner
RecyclerView isn't actually scrollable (that is, the contents of it are
not scrolled within it, the RecyclerView itself is scrolled by the outer
ScrollView instead). View.canScrollVertically is unfortunately incorrect
for RecyclerView in this case as well.

This change excludes all children of the main ScrollView from the
search. This  ensures the outer ScrollView is chosen and this corrects
the 'capture more' screenshot functionality on the Settings home page.

Bug: 399810823
Test: manual; Open settings, screenshot, capture more
Flag: com.android.settings.flags.extended_screenshots_exclude_nested_scrollables
Change-Id: I3363d70b3649e2a34e541c45ab387d59547b8588