Fix bug with launching non-50-50 splits in landscape on phones

This CL makes it so that DividerSnapAlgorithm.setDivideRatio() sets the middle position as a fallback when trying to fetch a non-existent snap target, instead of crashing.

The issue arose because it is possible to have a 30-70 split in Overview, then rotate to landscape, where 30-70 splits are not supported. Trying to launch the pair then resulted in a crash.

This change matches the behavior we already have when rotating a fullscreen 30-70 split to landscape, which is that the 30-70 split automatically converts to a 50-50 split.

Fixes: 326336840
Flag: N/A
Test: Does not crash anymore.
Change-Id: I68072c2816d5f6cf3a178b0f8def27cdc564d5d7