Replace ArrayList with a CopyOnWriteArrayList

RegularMaintenanceJobService use same ArrayList on both
omComplete and onStopJob methods, it's unsafe and will cause
ConcurrentModificationException when reading and removing
value at same time. Thus, replace ArrayList with
a CopyOnWriteArrayList.

Bug: 161205152
Test: atest NetworkStackTests
Change-Id: I3914e9ddc05e37d4deaa8846d7c3ba742160a788
Merged-In: I3914e9ddc05e37d4deaa8846d7c3ba742160a788
(cherry picked from commit 389735f7, aosp/1366998)