Respect the enforce flag for callback registration.

Predictive back infra respect the enforce flag everywhere else (e.g. for
compat / default callback registrations), so we follow the same pattern
here.

This flag is a development only flag, so this CL should not affect
droidfood or public builds.

Test: `adb shell setprop persist.wm.debug.predictive_back_always_enforce
1`. Restart. Check apps can register callbacks without opting in (e.g.
   can swipe back to LS from bouncer)

Change-Id: I7d5a85b37b50b11f6e4be28d022dd0e528a7d4fd