Don't add any trust agents if the controller is not available.

TrustAgentListPreferenceController gets a list of trust agents and add
them to the security_category preference category at runtime. However,
this update process is always ran - even if the controller is explicitly
disabled/not available. This fixes this.

Bug: None
Test: Updated TrustAgentListPreferenceControllerTest.
Change-Id: Ic95d219bc1d0c7cfa0c4eed0d3dd12dec0e14df6