Get keyguard state from UserManager lock state for bg users

If a visible background user is secure, the TrustManagerService will
always assert they are showing keyguard. However, this may not always be
the case - should the user be unlocked, the keyguard is no longer
showing and the TrustManager should be updated to reflect this.

Bug: 365626480
Test: atest FrameworksMockingServicesTests:TrustManagerServiceTest
Flag: NONE bugfix
Change-Id: Iee29496bbc23e9e440c9e59592e77c19f8757f13