docs: Corrected method name in multi-window docs

Confirmed on nyc-dev, the method name is
Activity.RequestDragAndDropPermissions(), not--as incorrectly
stated--Activity.RequestDropPermissions(). Updating preview docs
accordingly.

See first comment for doc stage location.

bug: 29057218
Change-Id: If147a9a5758abbe8fc46a6b40720e1328c78a095