Save app pair with 50-50 snap target if snap position is SNAP_TO_NONE

It was not possible to save an app pair if free snap mode was enabled
and the snap position was SNAP_TO_NONE. Just default to 50-50 split if
the current divider snap position is SNAP_TO_NONE.

Cherrypicked from isak.lilja@sony.com's CL at https://partner-android-review.git.corp.google.com/c/platform/packages/apps/Launcher3/+/2835425.

Bug: 343219704
Test: N/A
Flag: EXEMPT bugfix
Change-Id: I62347ac13440f714a493366ac66e893578a2d8d9