Fix NPE when rotation after split dismissed

Because drag to dismissed will not call SplitDecorManager#onResized
so mShown status will be wrong. Reset some flag in release and also
add some null check to avoid NPE.

Also refine some function params and naming to avoid confuse.

Fix: 226551765
Test: follow bug repro step and check no NPE
Test: pass existing tests
Change-Id: Ife592e176ef67c5df79be0a81509531ffcbd1b39