launcher: correct page spacing for multiple panels

This fixes 2 things:
- adding the correct space between pages as insets (eg camera) were
being discarded
- adding the space between pages instead of between panels as was before

This solution should work with more panels and don't create problems for
phones, where panels = 1.

Fixes 193194192
Test: manual testing in unfolded state in portrait and landscape, both
rotations

Change-Id: Ia3b148ceb773c6d5b6f8848ced07d7f9c1459e92