Implement CACert queries in SecurityController

Queries are run (on a AsyncTask) when user is switched and when
ACTION_TRUST_STORE_CHANGED is broadcasted. Otherwise, the result is cached
in the SecurityController.

Bug: 37535489
Test: runtest --path frameworks/base/packages/SystemUI/tests/src/com/android/systemui/statusbar/policy/SecurityControllerTest.java

Change-Id: I3b9cc3d85c9f49d0a892613b63d1fba184ab647e