Fix BroadcastReceiver leaked during user switching

Switch between two users multiple times will not continously increase
number of receivers registered to system, i.e. reach an equilibrium

Bug: 224501318
Test: check with adb:
while true; do adb shell dumpsys activity broadcasts | egrep \
'app=[0-9]+:system' | wc -l; date; sleep 1; done

Change-Id: Ifcf39250c4874cd0b7997c0fcbc4b85cd8a2f4f6