[DocsUI] use classifySelectionHotspot method

The recyclerview-selection library's inSelectRegion method (which
returns a boolean) was deprecated. We should implement
classifySelectionHotspot (which returns an int) instead.

The new method's two int values SELECTION_HOTSPOT_OUTSIDE and
SELECTION_HOTSPOT_INSIDE_TOGGLE_MULTI are equivalent to the old method's
two boolean values false and true.

classifySelectionHotspot can return other int values (like
SELECTION_HOTSPOT_INSIDE_TOGGLE_SOLO), but that is left for a follow-up
commit. This commit makes no behavior change. It only migrates from the
older return-boolean to the newer return-int method.

Bug: 389814214
Flag: EXEMPT no behavior change
Test: atest DocumentsUIGoogleTests
Change-Id: I3d86ac39578dcd5329f8b4ab66385d029fb6d04d