Always returning the updated preference summary

AppEntry is not being updated when opening nested app info activities,
which led to stale preference summary being returned for external
sources details.

Test: Manually checked the preference summary is correctly reflecting
the state of the switch.
Existing test ExternalSourcesSettingsTest passes.

Bug: 37487637
Change-Id: Id3c665c7cc7231023f4110725b3b15e43284de97