Fix "X unused apps" string to match w/ unused apps

This fixes an issue where there is some discrepancy between the "X
unused apps" string in Settings and the actual number of unused apps in
PermissionController due to the Settings logic not taking into account
the usage time.

Bug: 187464720
Test: atest HibernatedAppsPreferenceControllerTest
Test: hibernate app and revoke permission, open app, number of unused
apps matches up

Change-Id: Ifc30280e582e4183c61d421dd5378d627a80baf9