Use ConcurrentHashSet for bypassListeners

Listeners can be updated/added/removed from
different threads. Use a multithread-friendly data structure
for listeners to avoid a ConccurentModificationException.

Test: atest KeyguardBypassControllerTest
Fixes: 394230633
Flag: EXEMPT bugfix
Change-Id: I9b3c03318deefb81d1a798f7bd2d10e06a5b670d