Flexible split: Change tooltip logic

The tooltip will now appear once the divider has been dragged out of its original zone, and stay shown even if dragged back to its original zone.

Previously, the tooltip would be hidden for its original zone no matter what, making it awkward when the user dragged away and then back.

Fixes: 421118222
Flag: com.android.wm.shell.enable_magnetic_split_divider
Test: Manual
Change-Id: I5f16058d39c49098957505ea1336ae0a9dddeffa