Fetch active trustagents from LockPatternUtils

LockPatternUtils is what drives the TrustAgentsPreferenceController, and
therefore should be used to determine how many active trust agents there
are. TrustAgentManager filters on whether preferences are displayable in
the advanced settings app, which does not represent if the agent is
actually active.

Test: make RunSettingsRoboTests
Bug: 217217034
Change-Id: I8cae74d322b8e0658aabfe7d4646e9a299af4934