Move insets freeze to visibleRequested change

Instead of doing freeze/unfreeze exclusively for legacy
transitions, just freeze the insets whenever an activity goes
invisible; then clear it whenever the the activity is visible.

This is compatible with shell transitions as well.

Bug: 179270750
Test: updated/added WindowContainerTests
Change-Id: Iae5025beaed7d70a4623b446a1bb73574d420663