Improve the latency of first entering Apps page

Root cause:
AppsPreferenceController will query the recent app usages twice on
the main thread.

Solution:
Set flag to ensure only triggering refreshUi() once when entering
the Apps page.

Also correct the preference key of apps.xml since we don’t have
duplicated preferences now.

Fixes: 183176038
Test: robotests & visual
Change-Id: Ia41ee1e4a1946b8122deca63e318d3915afcc426