Allow custom keyguard transient message

TrustAgentServices can now present a transient
message on the lock screen or AoD.

Bug: 63940122
Test: call TrustAgentService#showKeyguardErrorMessage via service,
      lock device, wait for message to show up.
Change-Id: I222118787a1afb526ce7c90d46c41d0f20d8d912