Add elements to existing mChannelsList instead of overwriting in setChannels

This means that mChannels, which is initialized as an empty list upon declaration, cannot be overwritten (thus cannot be null) by a call to setChannels. This passes through the same channels in the list but does not keep the passed in structure.

Bug: 381131846
Flag: EXEMPT non-behavioral change
Test: NotificationManagerTest, NotificationManagerServiceTest, NotifictionChannelGroupTest
Change-Id: If8f564012b4000109734abab2339f776112cd146