Make TvInputManagerService robust against user race conditions

If USER_REMOVED was received by the broadcastreceiver before
USER_SWITCHED was processed, TvInputManagerService before this
change would null pointer and bring down the whole system server.

Change-Id: I004d549572e17aba05d0a8f3cf5c08d14391789d
Test: atest --iterations 20 WindowStateTests StartActivityAsUserTests
Fix: 149013745