Move unused apps count calculation to bg thread

Move the somewhat expensive calculation of the "Unused apps" count to
the background thread

Initially, the "Unused apps" preference is unavailable. When the bg work
finishes and we see we have a non-zero number of unused apps, we display
the preference and update the summary text.

Bug: 187996287
Test: atest HibernatedAppsPreferenceControllerTest
Test: measure latency of displaying preferences w/ custom trace points

Change-Id: Idb0d836fd8f4bcdd2605a7d59703a7ed53bcd6d4