Replace my new API with an existing one.

jsharkey pointed out an existing way to get PackageInfos for different
users, so let's use that instead.

Bug: 119811056
Test: Setup work profile, use both Contacts apps, give them different
permissions, then see separate entries in PermissionUsageFragment and
verify that AppPermissionFragment can change either app's permissions.

Change-Id: Ibe3ce8c191cda689fc6633d2a135f37342e3b2e8