Fix talkback issue about announcing wrong number of items

From hierarchical view, one of root cause is PreferenceCategory.
Looks like PreferenceCategory generates an invisible space when
we add a footerPreference as a child.

So, we remove PreferenceCategory directly, and declard "allow divider"
attribute for neccessary preferences.

Test: Visual
Fixes: 130396913
Change-Id: I6d56ac165633aa211e794e07417821066380d041