Fix and add unittests for SecurityController

testCaCertLoader was flaky because a background task started in
its constructor sometimes finished before the callback could be registered.
With this change, the callback is registered in the constructor.

Test: runtest --path frameworks/base/packages/SystemUI/tests/src/com/android/systemui/statusbar/policy/SecurityControllerTest.java
Bug: 37535489
Bug: 38045871

Change-Id: I4f5de1e142cb6e892132d804658c9b89e558144f