Update home bounds when entering split on rotation

Sometimes the rotation call comes in before split
is fully entered. However, we already know that we
are going into split-mode, so use a partial split
check rather than the full check to determine
whether home should be updated.

Also fixes another race where the incorrect position
was used to find closest snap on rotate before that
snap was reset to middle when leaving split (due to
a previous change that sets mMinimized early).

Bug: 156491146
Test: make various attempts at entering split by
      opening apps in a different orientation.
Change-Id: I380653e47dc0d07488c3ac56d39c4571d2f25f0b