Use StateMachine.sendMessage() to avoid NPEs on state machine quit

Bug: b/128913231
Test: Manual, on automotive hardware create two users, each with a
paired phone with phone back access enabled. Switch users several times,
making sure each switch shows the proper contacts. Switching forces the
state machine to shut down under one user and another start up in its
place.

Change-Id: I256730662e56a8511520ea915f99210f27b184cb