[DocsUI M3] Add bottom padding for file area

In compact screen size, the file area (list/grid) occupies the
full screen height, since in M3 we move breadcrumb down to the
bottom, we need to make extra bottom padding for the file area
to make sure the breadcrumb doesn't over-shadow the last item of
the file area.

Check the attached bug for demo video.

Bug: 397511300
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I2992ef65ec8db7deb77957d777c05bedc0ed4a32