Fixed booting logic on ActivityTaskSupervisor.activityIdleInternal()

It was clearing the list of users being switched when the device is
"booting", which could be a problem on systems running on headless
mode: if the method was called by a non-null record (like the
FallBackHome) before it was called by null, the non-system user would
never finish switching.

Test: manual verification # on automotive and phones

Fixes: 190854171
Bug: 203885241

Merged-In: I4c589fd618b104689434bf918ebb48a64cef30e4
Change-Id: I4c589fd618b104689434bf918ebb48a64cef30e4
(cherry picked from commit 1a6d740c)