Replace ALL_LISTS policy with JUST_WARN.

It seems pretty unlikely that we'd ever want to disallow access to the
light greylist in P, since doing do would break do many apps. We don't need
this policy here as an opt-in for apps now, since the StrictMode work will
achieve the same thing.

Instead, make a "just warn" policy which allows access to all APIs, but
leaves the detection and logging logic in place. This gives us the option
of disabling enforcement, but still gathering logs to find out which apps
use which APIs.

Bug: 77517571

Test: Boot device
Test: Hardcode policy of HIDDEN_API_ENFORCEMENT_JUST_WARN and verify log

Change-Id: I588f347716a79ac5887b74763c8afc16b3be699b