Tweak refresh of notification channel group preferences

Previously, we were discarding and re-adding all channel preference switches when a group was toggled. This caused the group switch itself to flicker and sometimes lose the a11y focus as well. Because the group toggle is always at the top, it's not necessary to ever re-sort it (which is the main reason for doing the remove+add dance).

This CL also resurrects ChannelListPreferenceControllerTest which was disabled.

Fixes: 248056830
Test: atest ChannelListPreferenceControllerTest

Change-Id: I5672c102d4571430e26ad6bee92d8c88794214ae