Autofill Settings: Display the number of saved passwords.

We bind to each available AutofillService, fetch the number of saved
passwords, then unbind and update the UI.

Each ServiceConnection is wired up to the controller's lifecycle so they
can be unbound when the lifecycle owner is destroyed.

Bug: 169455298
Test: manual - check value in the UI
Test: manual - no ServiceConnection leak, even when there's no response
Test: atest \
 SettingsUnitTests:com.android.settings.applications.autofill.PasswordsPreferenceControllerTest
Change-Id: I7008e979e9292b99c8611010e49b3e738c82bfed