The big keyguard transition refactor (2/n)

Introduce UnknownVisibilityController, which keeps track of apps that
may or may not be visible when launching an activity behind Keyguard.
When Keyguard is occluded and we launch another activity, we don't
know whether we still have to keep Keyguard occluded until the app
has gone through the resume path and issued a relayout call to update
the Keyguard flags.

This class keeps track of that state and defers the app transition
until the unknown visibility of all apps is resolved.

Test:
1) Have an occluding activity that starts another occluding
activity, ensure that there is no flicker.
2) Have an occluding activity while the Keyguard is insecure, start
a DISMISS_KEYGUARD activity, ensure there is no flicker.
3) runtest frameworks-services -c com.android.server.wm.UnknownVisibilityControllerTest

Bug: 32057734
Change-Id: I5145b272722ab8c31dd7c5383286f5c9473e26a4