Extract counting of installed apps into helper class

This CL extracts the code that counts the number of installed apps
for a set of users from ManageApplications to a helper class,
InstalledAppCounter. The new helper is designed to be unit-testable
(PackageManager dependencies are injected via a new PackageManagerWrapper
abstraction) and customizable (the set of users for which to count is
specified by subclasses).

The CL adds a unit test for the InstalledAppCounter as well.

Test: make RunSettingsRoboTests
Bug: 32692748
Change-Id: Ie7fbd40d9dd998b27f1f9de227b6a187b5c9d315