Fix 2 issues related to caption.

1. This change ignores immersive mode request if the window is not in
fullscreen mode in terms of caption visibility;
2. Update caption visibility when app changes system UI visibility.

This change also consolidates the visibility logic into DecorView.

Bug: 119388201
Test: Manual test. Also test that split screen and PIP don't have
caption.

Change-Id: Ia5051d2f53455be2c21e1933b002d07b25c0b74b