Set snapTargetBeforeMinimized to middle target if position is negative

There are cases where the last snapTarget position is negative.
Specifically, it can be negative if the divider was moved up, making the
last position negative off-screen. Instead save the middle SnapTarget,
which is the default, so bounds aren't accidentally calculated with
negative position.

Fixes: 74440003
Test: Bug no longer occurs
Test: Split screen with 3rd party launcher
Test: ActivityManagerSplitScreenTests

Change-Id: Id3261d6e31622a96f71ff80301dd9cc39599b7ec