[Flexiglass] Fix 1px rounding error

In some cases, in split shade in flexiglass, due to rounding errors, the
leftmost 1px of tiles in the second page can be seen peeking from under
the notifications scrim. This is due to the fact that rounding errors
accumulate when calculating the page margins for the PagedTileLayout.

With this change, when in flexiglass and if the page margin is expected
to be half of the total margin, but the total margin is odd, instead of
having 0.5px missing on each side, we round down in one side and up in
the other.

Test: manual
Fixes: 339863138
Flag: com.android.systemui.scene_container

Change-Id: I1bcf9b730c9e70056bc3022558bf1045b30d3a60