Let system bar visibility override can be cascaded

If there are multiple callers overriding system bar visibilities, the
overrides should be cascaded. So that if a caller revokes its own
override, overrides of others can still take effect.

This CL also revokes the override if the caller dies.

Fix: 407898759
Flag: EXEMPT refine new infrastructure
Test: atest DisplayPolicyTests WindowOrganizerTests
Change-Id: I6b9644087ede15b25f9cbe46fe43e802b1382e1a