Respect StatusBarManager back/home disable flags

* Also use the same signals provided by SysUi flags
instead of directly from the disable API just for
consistency. (Disable2 flags still need to be migrated
over, so we're keeping the API for now).

Bug: 194870114
Test: Locking screen w/ 3 button nav
w/ various keyguards

Change-Id: I858ea36876061cddf0debbd43e8fcfe1d5712839