Change SnapPosition numbering for app pairs rank encoding

This patch changes the specific ints asssigned to the SnapPosition enum (we use them to encode split information in app pairs). Now there is a new subset IntDef, PersistentSnapPosition, for ints that are passed up to launcher for use with launching splitscreen apps and saving app pairs.

Bug: 274189428
Test: Existing tests still pass
Change-Id: Ibdbdff3e8101829cdd29068fbf278a0f9760606a