Add aconfig flag for enable_magnetic_split_divider

Used for flagging some new divider changes. This feature makes the split screen divider snap 'magnetically' to available snap points as the user drags it.

Bug: 383631946
Flag: com.android.wm.shell.enable_magnetic_split_divider
Test: Not needed
Change-Id: I3073ffdb7b8e5bc0e07c903076625b1036ed2937